I was plagued by RenDev line 547 errors when using the Unreal Tournament OS X Beta and did some searching around for possible fixes. Didn't find anything concrete, so went onto my own. Turns out that the UnrealTournament.ini file which resides at the root level of the Unreal Tournament folder is the culprit and it is OpenGL related.
If you are using UT for OS X and run into problems, try trashing the UnrealTournament.ini file (this is the OS X replacement that comes with the OS X UT game file), then replace it with a fresh copy of the UnrealTournament.ini file. But here is the fix -- be sure to lock the file before you run UT again. I haven't had any problems since replacing and locking.
Hope this helps someone.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030227102836425