I tried to create a new branch with a name that has an exclamation mark(!) at the end. Sourcetree will tell me this is an invalid branch name.
If I try to checkout a branch with an exclamation mark at the end, sourcetree does not let me and displays a "invalid remote branch" message.
Trying the same on a Windows machine works.
I don't believe the exclamation mark is an invalid character for branch names according to the git documentation.
I'm using the lastest build on Mac 4.1.6(242)