This happens periodically, with no predictability. When cutting and occasionally copying text from a file using visual mode or a mouse select (I assume these are equivalent,) MacVim crashes and the window freezes. Other windows are still accessible. I presume this is because each window is its own VIM process.
The following error is dropped in the console:
5/20/11 3:50:24 P [0x0-0x8c48c4].org.vim.MacVim Vim: Caught deadly signal SEGV
5/20/11 3:50:24 P [0x0-0x8c48c4].org.vim.MacVim Vim: preserving files...
It appears to be some sort of memory allocation issue. Perhaps memory is asked for, not granted, and not handled gracefully (like trying again?)