I upgraded MySQL on Uranus this weekend from 4.0.x to 4.1.13.  What I didn't know until I got phone calls about a website not working is that MySQL 4.1 no longer supports the ISAM tables by default.  Some of my databases for the websites have been around since 3.x and are still using that table type that is being phased out.  So now I have to recompile MySQL 4.1 so that I can convert the tables over to the newer format so I can get one with my day.  *grumbles*