If one has the "Auto-Save Successful Replays" option enabled, and uses the cheat key to skip a level, a replay will nonetheless be saved for it. On some older versions, this may have the "FAILED" tag next to it (but is still saved nonetheless), but on newer versions (possibly only the latest experimental ones) they're indistinguishable from genuinely successful replays, except by actually opening them and seeing if they solve the level.
