Which build infrastructure problems do you see the most?

I'm hoping to tackle bug 505512 (Make infrastructure related problems turn the tree a color other than red) in the next few weeks. Most of the ground work for it is laid, which means that most of what I'll be doing is parsing logs for infrastructure errors.

So, what errors do you see most from our build infrastructure? Are there other things that you would classify as infrastructure issues? Please add any suggestions you have to this Etherpad: http://etherpad.mozilla.com:9000/build-infra-errors