First of all, I'm not sure whether I should create this issue here, or directly to vim bug tracker, but other omni completion methods (ie. syntaxcomplete#Complete) work fine.
Steps to reproduce:
1. start vim
2. enter :setf ruby<cr>
3. enter iSTDIN<c-x><c-o>
Pop up a list of completions.
Vim: Caught deadly signal SEGV
Vim: preserving files...
 16018 segmentation fault vim
The vim version I'm using is 7.3.762 (installed from Homebrew on OS X Lion).
I think I've tracked the issue myself. It looks like vim grabs wrong ruby (I have multiple rubies installed via rbenv). Sorry for inconvenience :)
Any guide how to resolve this problem? I use rbenv too.
General answer is to make sure you're using the system ruby when compiling Vim.
I used this guide: http://andrew-stewart.ca/2012/12/23/vim-ruby-1.9.3 and compiled against 1.9.3 (not system one).
It works also for 1.8.7 projects, so I won't recompile it again ;)
I think more general rule is to ensure the ruby in your $PATH is the same as in vim --version.
For example now mvim segfaults when executed from terminal, but not if executed from /Applications. Unfortunatelly vim uses ruby from $PATH, not the one it was compiled with. @tpope maybe implement this feature in vim-ruby?
OK. Now python screws things up. I guess you should always compile against system ruby. In that case this should be probably modified by homebrew, not vim-ruby.
I stil get this issue - using Vim ( not MacVim ) though I am on a Mac.