I’m on Ubuntu 10.04 and running the latest beta (5?). I have found this in all the betas and I think in the stable version. If you import a large sample (say a whole song so around 5 mins ~10MB mp3 file) and then import another sample in a new slot before the first conversion has finished, the conversion of the first sample halts at the point you started to import the new sample. Also, if you switch to the sample editor screen and switch quickly between the first imported but halted sample and the second sample, it will eventually freeze Renoise.
Renoise doesn’t support mp3 import natively (because of license matters) but uses third party libraries. These third party libraries have limitations though but not all are known.