I've been looking at this thread on and off but haven't noticed this problem before (don't shoot me if it's a repeat!):
On Friday morning (in Aus) I tried joining a friend's private server for a league we race in, only I was getting an odd error saying I needed patch U (there was a number also, but I didn't screenshot it).
At the time I figured the host must have the new test patch 20 installed and it had compatibility issues, so I quickly installed it and I was able to connect perfectly ok. Now I was previously using test patch 19, and the server and most of the people in it were not using any test patches at all. I'd never had a problem like this before, even with test patch 19, so I'm at a loss as to why this happened. I know it's not a TP20 problem, as it seems to work now, but it does seem a bit of a worry and you might want to know about it for future patches...
so in brief - patch 19 had a (once off) incorrect version error when joining a standard S2U server, patch 20 fixed the problem.
Anyone else have this problem before?