YouCompleteMe uses tab for completion, and so does SnipMate. If you want to use both, you need to change SnipMate-s trigger key using:
tab
SnipMate
imap <C-J> <esc>a<Plug>snipMateNextOrTrigger smap <C-J> <Plug>snipMateNextOrTrigger
Ref: https://github.com/Valloric/YouCompleteMe/issues/47#issuecomment-20529309
Deleting the wiki page "Known Conflicts between other plugins" cannot be undone. Continue?