diff --git a/scripts/backup/files/backup_files.sh b/scripts/backup/files/backup_files.sh
index bc6b1c8..8674675 100644
--- a/scripts/backup/files/backup_files.sh
+++ b/scripts/backup/files/backup_files.sh
@@ -89,11 +89,15 @@ backupFilesCapture()
         # docker must go through runFileOp: on rootless installs the daemon
         # socket lives in the install user's runtime dir and only the wrapper
         # sets DOCKER_HOST for it — bare `docker` can't connect at all. Keep
-        # stderr in a scratch file so a failure can say why.
+        # stderr so a failure can say why — one file per pipe half: sharing a
+        # file mixes an O_TRUNC fd with an O_APPEND one and the halves race,
+        # overwriting each other's lines.
         local err_file capture_status
         err_file=$(mktemp "${TMPDIR:-/tmp}/lp-capture-XXXXXX.err")
-        runFileOp docker exec "$container" tar -C "$cpath" -cf - . 2>"$err_file" | runFileOp tar -xf - -C "$stage" 2>>"$err_file"
+        runFileOp docker exec "$container" tar -C "$cpath" -cf - . 2>"$err_file.docker" | runFileOp tar -xf - -C "$stage" 2>"$err_file.host"
         capture_status=("${PIPESTATUS[@]}")
+        cat "$err_file.docker" "$err_file.host" > "$err_file" 2>/dev/null
+        rm -f "$err_file.docker" "$err_file.host"
         if [[ ${capture_status[0]} -eq 0 && ${capture_status[1]} -eq 0 ]]; then
             # The capture preserves the app's ownership (e.g. www-data, 0640),
             # which the backup user still couldn't read. Hand the staging tree to
@@ -119,7 +123,11 @@ backupFilesCapture()
 
             isSuccessful "captured $subdir ($(du -sh "$stage" 2>/dev/null | cut -f1))"
         else
-            if grep -q "executable file not found" "$err_file" 2>/dev/null; then
+            # docker exec exits 126/127 when the binary can't start — and its
+            # "executable file not found" message goes to STDOUT (a docker
+            # quirk), i.e. into the pipe, so the exit code is the only
+            # trustworthy signal.
+            if [[ ${capture_status[0]} -eq 126 || ${capture_status[0]} -eq 127 ]]; then
                 isError "capture of $subdir from $container failed — the image has no tar, so live capture cannot work for this app"
             else
                 isError "capture of $subdir from $container failed:"
@@ -189,10 +197,12 @@ restoreFilesRehydratePreStart()
         # docker goes through runFileOp for the rootless socket, same as capture.
         local err_file restore_status
         err_file=$(mktemp "${TMPDIR:-/tmp}/lp-rehydrate-XXXXXX.err")
-        runFileOp tar -C "$stage" -cf - . 2>"$err_file" | runFileOp docker run --rm -i \
+        runFileOp tar -C "$stage" -cf - . 2>"$err_file.host" | runFileOp docker run --rm -i \
                 -v "$app_dir:/parent" "$backup_files_helper_image" \
-                sh -c "rm -rf '/parent/$subdir' && mkdir -p '/parent/$subdir' && tar -C '/parent/$subdir' -xf - && chown -R $uid:$gid '/parent/$subdir'" 2>>"$err_file"
+                sh -c "rm -rf '/parent/$subdir' && mkdir -p '/parent/$subdir' && tar -C '/parent/$subdir' -xf - && chown -R $uid:$gid '/parent/$subdir'" 2>"$err_file.docker"
         restore_status=("${PIPESTATUS[@]}")
+        cat "$err_file.host" "$err_file.docker" > "$err_file" 2>/dev/null
+        rm -f "$err_file.host" "$err_file.docker"
         if [[ ${restore_status[0]} -eq 0 && ${restore_status[1]} -eq 0 ]]; then
             isSuccessful "restored $subdir"
         else