No personal experience but someone blogged (in German, unfortunately) recently about a bug in 23andMe's analysis algorithm that resulted in him being told that he was terminally ill. He spent a lot of time learning everything about the purported gene malfunction, found the bug in 23andMe's assumptions and got it fixed upstream. Certainly not a nice experience.

[1] http://www.ctrl-verlust.net/23andme-wie-ich-fur-todkrank-erk...