We've recently upgraded to SMF RC3. This cost us all custom modifications to the SMF forum system. This is what's broken, that we are currently aware:
1) Registration. I implemented a regexp captcha to the SMF engine because its original system was too poor to handle multiple answers. My system allowed them to be set as (answer1|answer2), or however else is legal in the regexp standard. As such, any question with more than one answer is impossible to answer correctly.
2) Unix names. Gary just implemented a system that allows users to give themselves a unix name. It wasn't implemented in the Wiki completely yet, but it's gone now.
3) Code highlighting. The system had a problem with generating previews (the page had to be refreshed to see any modifications), and it didn't distinguish between GML and C++, but it's gone now.
Any other problems, report them here.