When I first flashed CM7 on my SGS, I had an issue with my sim card and had to revert to stock. However, Odin got stuck and I my phone refused to go into download mode (see my comment in this question for some details). When I finally got it to go into download mode, I did the same as before but without re-partitioning, and it worked.
Does the fact that Odin got stuck on me last time I came from CM7 mean that I did anything wrong? Should I have done anything differently due to the fact that I came from CM7?