I have to say that oftentimes, I have bathroom-related OCD. Especially when I am a guest somewhere. I am always afraid that the door is not locked. And I always double- and triple-check it. Complicated locks are not my friends. The locks that open automatically once I turn the knob from the inside are the worst. I can never be absolutely sure that the door is locked, unless I ask someone outside to try opening the door and witness him failing (Karl Popper would be happy) — which, of course, I never do.
Which is all why I agree with Ilya Birman that this design for a bathroom lock is the best, as it leaves no doubt as to whether the door is locked (and how securely):
(On the other hand, one time, while staying with my fiancée’s uncle and aunt, I couldn’t open the bathroom for a few minutes — the door wouldn’t open whichever way I turned the lock. By the time I managed to open the door, I started thinking I might have to spend the whole Shabbos in the bathroom.)