When you leave the checkin comment box empty and try to commit your changes, a question box appears asking you if you really want to continue. At that point you can either click "Yes" or "No". But there is a third option: closing the window using the "X" in its right top corner (or pressing the Alt + F4 key combination, for instance). When you do that, something unexpected happens: the checkin operation is continued instead of being cancelled.

IMHO it should be impossible to close the window by other means than clicking the Yes or No buttons. Or, if that is allowed, closing the window should cause taking an action being "safer" for the user. In the above mentioned case the commit operation should not be started.

I'm pretty sure that the Pending Changes view is not the only place where the problem can be encountered, but I can't tell you now where else I have seen it as well.