diff --git a/harness/badapple/README.md b/harness/badapple/README.md
index c890a77..4bcdc43 100644
--- a/harness/badapple/README.md
+++ b/harness/badapple/README.md
@@ -208,6 +208,8 @@ path; for a live run the screen is the evidence.
 | symptom | cause |
 |---|---|
 | `demo.sh: ... did not answer within 180s` | the terminal is not on the tap, or `run.sh net` was started without the LANCE interface selected in its cfg |
+| `demo.sh: ... has no dictionary.bin` | `--tiles` wants a **cache** directory, not the frames directory `clip_from_video.py` wrote; the message names the `make_tiles.py --clip … --out …` command that builds it |
+| payload already jumped, need to send again | the script refuses a bad cache path *before* the jump for this reason, but if it happens: rerun with `--no-load` — the payload is still running and waiting, and `vidsend` brings the dictionary with it |
 | poke reads back a mismatch | the image is being loaded over a stale link, or the payload was built for a different load address |
 | `DICT_BYTES` stuck below 32768 | datagrams lost before the payload saw them — check the tap and the sender's interface |
 | picture sheared in MAME | `VT1200_MONITOR=VR150` is selected (the old derived 1024-px layout); leave the default VR319 |
diff --git a/harness/badapple/demo.sh b/harness/badapple/demo.sh
index 9c25216..bfe795c 100755
--- a/harness/badapple/demo.sh
+++ b/harness/badapple/demo.sh
@@ -42,6 +42,25 @@ while [ $# -gt 0 ]; do
     esac
 done
 
+# --- the cache, checked BEFORE anything irreversible ------------------------
+# The jump hands the payload the remote-debug process and it never returns, so
+# a bad --tiles path discovered afterwards leaves the machine needing a restart
+# to try again.  Refuse it here instead.
+if [ "$SEND" = 1 ]; then
+    for f in dictionary.bin stream.bin; do
+        if [ ! -f "$TILES/$f" ]; then
+            echo "demo.sh: $TILES has no $f -- that is not a tile cache" >&2
+            if ls "$TILES"/[0-9]*.bin > /dev/null 2>&1; then
+                echo "         it looks like a FRAMES directory (raw 1-bpp frames)." >&2
+                echo "         build the cache from it first:" >&2
+                echo "           python3 harness/udpstream/make_tiles.py --clip $TILES --out $TILES-tiles" >&2
+            fi
+            echo "         the committed 30-frame cache is harness/udpstream/tiles" >&2
+            exit 2
+        fi
+    done
+fi
+
 # --- readiness: an ICMP reply from the terminal's own stack -----------------
 start=$(date +%s)
 while ! ping -c1 -W1 "$HOST" > /dev/null 2>&1; do