This replay does not reproduce the issue for me on RC5 either.
Can you give any additional information? Don't worry if you can't answer all of these - any info is useful.
- Were you playing via normal level select, or Test Play mode from the editor?
- Had you been playing for a long time, or was this the first (or close to the first) level you played during that session? If you had been playing for a long session, did you notice any worsening lag and/or slower load times as the session went on?
- Since the last time you had been on the preview screen, had you restarted the level (via hotkey / skill panel button, not via exiting to postview), performed a backwards framestep, or loaded a saved state?
- Do you remember anything else in particular you were doing at the time? Eg. scrolling the level, using Clear Physics Mode, attempting to mouseover a lemming?
- Had you done anything outside of standard gameplay, shortly prior to this? This could include saving level images, a mass replay check, running a Cleanse Levels, etc.
- Was the game paused at the time?
- Anything else you can think of that might be helpful?
This could very well end up being a "needle in a haystack" bug with extremely specific conditions, so any information could be useful. Even if you're in doubt as to whether it's useful or not - the worst that can happen is that it doesn't achieve anything; it can't hurt. I won't delay 12.10 stable beyond tomorrow if I can't get any solid leads on this bug, unless I get additional reports that would suggest it's a frequent issue. The info in the spoiler tag suggests it may be related to a bitmap image in memory being loaded, created blank, or resized (as that's usually where DIB handle issues come up), though this is far from a sure thing.