I installed YCM as per the instructions on the readme, and now MacVim crashes immediately after starting.

Running it in the terminal I get—

Fatal Python error: PyThreadState_Get: no current thread
Vim: Caught deadly signal ABRT
Vim: Finished.

I'm running MacVim 7.3 (snapshot 66) for OS X 10.8 (Mountain Lion), as downloaded from the official Google Code project page.

Running plain old Vim (which IIRC I brew installed), I get—

Vim: Caught deadly signal ABRT
Vim: Finished.
Abort trap: 6

When I ran :BundleInstall Valloric/YouCompleteMe, MacVim didn't crash—but then I'm not sure if it required an reboot to fully "install".