F.e. in contacts, new contact. Focus on 'Phone', now blur, focus on 'Name', the "number" keyboard is visible. We should never pop up the new keyboard if the new keyboard is not done rendering. I fixed it in a branch (https://github.com/comoyo/gaia/compare/keyboard_perf_863662?expand=1) but it requires quite some control flow changes. Newly introduced with the new IME changes.
Jan, thanks for filing this bug. I guess Bug 910697 can be used to track this.