One of the contractors I spoke with said that they had a branch running Rails 2.1 successfully. When they deployed it in production, the entire application fell on its face.<p>Supposedly, the problem was caused by Cache Money, but nobody at Twitter wanted to risk moving to a different version again. They're still on 2.0 today. :-)<p>Another fun fact: Twitter has over 1,500 remote git branches. They also have bright green deer in the reception area of their office. :-)