I am also getting this error message with my premiere pro CS5.5. Is there a resolution to this issue yet?
I think I found the answer to this error SequenceAddTransitionAction.cpp-57.
I placed a .wav file and audio from a .mp4 on the same track and I got the error message.
When I moved the .mp4 audio to a different track the system was stable.
I also rendered out any audio transistions for good messure.
I've come to expect problems with compressed audio in a Timeline. I'm amazed when problems don't crop up.
Some times, I'll toss the mp3 sample clips from a stock library into a Timeline to see how it works against the picture, or to get through the client review process, before I purchase the AIF file.
Pr CS5.5 Mac has been pretty good so far with .mp3 and AAC files (and also mixed sample rates). But, when I do see weirdness with compressed audio formats, I'll usually transcode them to 48KHz WAV or AIF, and the audio problems usually vanish.