Watch out for this one - I almost didn't notice it. Today I went to add a new mapping. It just so happened that the mapping I added was one that already existed. The administrator correctly noticed this and gave me an error:
No big deal, right? I'll just rename mine to /org2 and hit submit.
Ouch. It actually replaced my org mapping. Not the end of the world, but if you had a lot of mappings it could easily be missed. Note - I recommend - almost always - that you use Application specific mappings. In my case though it is a code base I can't modify just yet.
I have filed a bug report for this, but unfortunately the public bug base still hides 901 bugs.