If I open a command window (search or command history) and try a window command (e.g. <C-w>p) I get an E11 error from Vim as expected. If I try to open CtrlP after that, it does nothing because it thinks it's in a command window. This happens because ctrlp#init calls s:iscmdwin which is supposed to not allow CtrlP to open if currently in a command window, but it is fooled if the last error before attempting to open CtrlP was an E11.
Temporary work-around is to map :let v:errmsg=""<CR>:<C-u>CtrlP<CR>