diff --git a/crates/ledger-store/src/journal.rs b/crates/ledger-store/src/journal.rs
index cdd4ac0..8e11f32 100644
--- a/crates/ledger-store/src/journal.rs
+++ b/crates/ledger-store/src/journal.rs
@@ -162,9 +162,21 @@ impl JournalWriter {
             prev: self.prev_hash.clone(),
         };
         let bytes = line.to_canonical();
-        self.file.write_all(bytes.as_bytes())?;
-        self.file.write_all(b"\n")?;
-        self.file.sync_all()?;
+        // A write that fails partway (a full disk) must leave nothing
+        // behind: cut the file back to where it was. Otherwise a partial
+        // line stays, and every later line lands after it, mid-journal
+        // (found by the full-disk fault test, 2026-10-13).
+        let before = self.file.metadata()?.len();
+        let written = self
+            .file
+            .write_all(bytes.as_bytes())
+            .and_then(|()| self.file.write_all(b"\n"))
+            .and_then(|()| self.file.sync_all());
+        if let Err(e) = written {
+            let _ = self.file.set_len(before);
+            let _ = self.file.sync_all();
+            return Err(e.into());
+        }
         self.prev_hash = sha256_hex(bytes.as_bytes());
         self.next_seq += 1;
         Ok(line)
diff --git a/crates/ledger-store/src/store.rs b/crates/ledger-store/src/store.rs
index 205331b..5cf7e52 100644
--- a/crates/ledger-store/src/store.rs
+++ b/crates/ledger-store/src/store.rs
@@ -3498,6 +3498,29 @@ pub(crate) fn apply_line_to_sqlite(
     conn: &Connection,
     line: &JournalLine,
 ) -> Result<(), StoreError> {
+    // All or nothing, per line: the rows, the events mirror, and
+    // last_applied_seq commit together. Without this a write that failed
+    // partway (a full disk) committed the rows but not last_applied_seq,
+    // and the next open's catch-up re-applied the line into a UNIQUE
+    // violation: the books would not open (found by the full-disk fault
+    // test, 2026-10-13). A savepoint, so it nests inside a caller's
+    // transaction.
+    conn.execute_batch("SAVEPOINT apply_line")?;
+    match apply_line_rows(conn, line) {
+        Ok(()) => {
+            conn.execute_batch("RELEASE apply_line")?;
+            Ok(())
+        }
+        Err(e) => {
+            let _ = conn.execute_batch("ROLLBACK TO apply_line; RELEASE apply_line");
+            Err(e)
+        }
+    }
+}
+
+/// The statements behind [`apply_line_to_sqlite`], run inside its
+/// savepoint.
+fn apply_line_rows(conn: &Connection, line: &JournalLine) -> Result<(), StoreError> {
     let p = &line.payload;
     match line.line_type.as_str() {
         "store.created" => {}
diff --git a/crates/ledger-store/tests/faults.rs b/crates/ledger-store/tests/faults.rs
new file mode 100644
index 0000000..7fec82a
--- /dev/null
+++ b/crates/ledger-store/tests/faults.rs
@@ -0,0 +1,205 @@
+//! Fault injection (rank 1): the store under failing I/O.
+//!
+//! A full disk is simulated without any seam in the code: the test re-runs
+//! this binary as a child process under a shell that caps the file size
+//! (`ulimit -f`) and ignores SIGXFSZ, so writes past the cap fail with
+//! EFBIG exactly as a full disk fails a write. The child appends until a
+//! write fails; the parent then checks the books.
+//!
+//! Its own file: the child mode needs a test binary of its own to re-run.
+
+use chrono::NaiveDate;
+use ledger_core::{AccountType, Currency, Money, Side};
+use ledger_store::Store;
+use rust_decimal::Decimal;
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+fn test_dir(name: &str) -> PathBuf {
+    let dir = std::env::temp_dir().join(format!("wanderbooks-fault-{name}-{}", std::process::id()));
+    let _ = std::fs::remove_dir_all(&dir);
+    std::fs::create_dir_all(&dir).unwrap();
+    dir
+}
+
+fn post_one(store: &mut Store, n: u32) -> Result<(), ledger_store::StoreError> {
+    let cash = store.ledger().chart().find_by_code("1000").unwrap().id;
+    let sales = store.ledger().chart().find_by_code("4000").unwrap().id;
+    let day = NaiveDate::from_ymd_opt(2026, 1, 1).unwrap() + chrono::Days::new(u64::from(n % 300));
+    let e = store.create_entry(day, &format!("sale {n}"))?;
+    let m = Money::new(Decimal::new(i64::from(n) * 100 + 1, 2), Currency::USD);
+    store.add_line(e, cash, Side::Debit, m)?;
+    store.add_line(e, sales, Side::Credit, m)?;
+    store.post_entry(e)
+}
+
+/// The child: append until a write fails, then report how far it got.
+#[test]
+fn child_append_until_the_disk_is_full() {
+    let Ok(dir) = std::env::var("WB_FAULT_DIR") else {
+        return; // Only runs as the child.
+    };
+    let mut store = Store::open(Path::new(&dir)).expect("child opens the books");
+    for n in 0..100_000 {
+        if let Err(e) = post_one(&mut store, n) {
+            println!("CHILD write failed after {n} entries: {e}");
+            // Keep going a little: later commands must fail cleanly too,
+            // and must not write after a partial line.
+            for k in 0..5 {
+                let _ = post_one(&mut store, n + 1 + k);
+            }
+            return;
+        }
+    }
+    println!("CHILD never failed");
+}
+
+/// Oracle (rank 1, docs/receipt-store.md: a failed write leaves nothing
+/// behind; R-11): when the disk fills mid-command, the command fails, the
+/// journal holds only whole lines, and the books open and verify with
+/// everything that was acknowledged. SQLite's write-ahead log is the
+/// first file to fill here; the journal-side case is tested on its own
+/// below.
+fn full_disk_case(name: &str, drafts: u32) -> String {
+    let dir = test_dir(name);
+    {
+        let mut store = Store::create(&dir, Currency::USD).unwrap();
+        store
+            .add_account("1000", "Cash", AccountType::Asset, None, false)
+            .unwrap();
+        store
+            .add_account("4000", "Sales", AccountType::Revenue, None, false)
+            .unwrap();
+        for n in 0..5 {
+            post_one(&mut store, n).unwrap();
+        }
+        let cash = store.ledger().chart().find_by_code("1000").unwrap().id;
+        for n in 0..drafts {
+            let e = store
+                .create_entry(
+                    NaiveDate::from_ymd_opt(2026, 6, 1).unwrap(),
+                    &format!("draft {n} {}", "x".repeat(200)),
+                )
+                .unwrap();
+            let m = Money::new(Decimal::new(100, 2), Currency::USD);
+            store.add_line(e, cash, Side::Debit, m).unwrap();
+        }
+    }
+    let journal = dir.join(ledger_store::JOURNAL_FILE);
+    // The cap applies to every file the child writes, so it must clear
+    // the biggest one with a little room: whichever grows past it first
+    // fails, as on a full disk. bash counts ulimit -f in 1024-byte blocks.
+    let biggest = std::fs::read_dir(&dir)
+        .unwrap()
+        .filter_map(|e| e.ok()?.metadata().ok())
+        .filter(|m| m.is_file())
+        .map(|m| m.len())
+        .max()
+        .unwrap();
+    let blocks = biggest / 1024 + 3;
+    let exe = std::env::current_exe().unwrap();
+    let out = Command::new("bash")
+        .arg("-c")
+        .arg(format!(
+            "trap '' XFSZ; ulimit -f {blocks}; exec \"$0\" --exact child_append_until_the_disk_is_full --nocapture --test-threads=1"
+        ))
+        .arg(&exe)
+        .env("WB_FAULT_DIR", &dir)
+        .output()
+        .unwrap();
+    let stdout = String::from_utf8_lossy(&out.stdout);
+    let child = stdout
+        .lines()
+        .find(|l| l.contains("CHILD"))
+        .unwrap_or("no child line")
+        .to_string();
+    assert!(
+        child.contains("CHILD write failed"),
+        "the cap never bit: {child}\n{}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+    let bytes = std::fs::read(&journal).unwrap();
+    assert_eq!(
+        bytes.last(),
+        Some(&b'\n'),
+        "the journal ends mid-line ({child})"
+    );
+    let store = Store::open(&dir).unwrap_or_else(|e| panic!("the books don't open ({child}): {e}"));
+    store.verify().unwrap();
+    assert!(store.ledger().posted_entries().count() >= 5);
+    child
+}
+
+#[test]
+fn a_full_disk_mid_command_leaves_the_books_whole() {
+    let db_first = full_disk_case("full-disk-db", 0);
+    assert!(
+        db_first.contains("sqlite"),
+        "expected the database to fill first: {db_first}"
+    );
+}
+
+/// The child for the journal-side case: the journal writer alone (no
+/// SQLite, whose write-ahead log otherwise fills first), appending until a
+/// write fails, then a few more appends.
+#[test]
+fn child_journal_appends_until_the_disk_is_full() {
+    let Ok(path) = std::env::var("WB_FAULT_JOURNAL") else {
+        return;
+    };
+    let mut w =
+        ledger_store::JournalWriter::open(Path::new(&path)).expect("child opens the journal");
+    for n in 0..1_000_000u64 {
+        if let Err(e) = w.append(
+            "test.line",
+            serde_json::json!({ "n": n, "pad": "x".repeat(300) }),
+        ) {
+            println!("CHILD write failed after {n} lines: {e}");
+            for k in 0..5 {
+                let r = w.append("test.line", serde_json::json!({ "n": n + 1 + k }));
+                println!(
+                    "CHILD retry {k}: {}",
+                    if r.is_ok() { "ok" } else { "failed" }
+                );
+            }
+            return;
+        }
+    }
+}
+
+/// Oracle (rank 1): a journal append that fails partway (a full disk)
+/// leaves no partial line: the journal holds only whole lines and its
+/// hash chain verifies.
+#[test]
+fn a_full_disk_mid_append_leaves_only_whole_journal_lines() {
+    let dir = test_dir("journal-full");
+    let path = dir.join(ledger_store::JOURNAL_FILE);
+    {
+        let mut w = ledger_store::JournalWriter::open(&path).unwrap();
+        for n in 0..20u64 {
+            w.append("test.line", serde_json::json!({ "n": n }))
+                .unwrap();
+        }
+    }
+    let blocks = std::fs::metadata(&path).unwrap().len() / 1024 + 8;
+    let exe = std::env::current_exe().unwrap();
+    let out = Command::new("bash")
+        .arg("-c")
+        .arg(format!(
+            "trap '' XFSZ; ulimit -f {blocks}; exec \"$0\" --exact child_journal_appends_until_the_disk_is_full --nocapture --test-threads=1"
+        ))
+        .arg(&exe)
+        .env("WB_FAULT_JOURNAL", &path)
+        .output()
+        .unwrap();
+    let stdout = String::from_utf8_lossy(&out.stdout);
+    assert!(
+        stdout.contains("CHILD write failed"),
+        "the cap never bit:\n{stdout}"
+    );
+    let bytes = std::fs::read(&path).unwrap();
+    assert_eq!(bytes.last(), Some(&b'\n'), "a partial line was left behind");
+    let lines = ledger_store::journal::read_and_verify_journal(&path)
+        .unwrap_or_else(|e| panic!("the journal doesn't verify: {e}"));
+    assert!(lines.len() >= 20);
+}