In e1ec57d this line was added:
And it probably fixed some issues in git-flow, but it causes breakages down the line. Specifically, when git-flow does git checkout, that can trigger post-checkout git hooks. Some people in Rubyland have hooks that integrate with some Ruby-specific tools, and have rbenv (written in bash) to manage them. Because rbenv uses a construct (process substitution) that's not posix-compatible, this environment variable causes it to fail.
This was discovered in sstephenson/rbenv#222
In summary, because there might be other shell scripts down the line, git-flow should not mess up the environment for them. I propose either removing the export, or using the setting:
set -o posix