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.

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. :-)

Another fun fact: Twitter has over 1,500 remote git branches. They also have bright green deer in the reception area of their office. :-)