Take the display configuration dialogue. It's window has a minimum size. If you decrease your resolution to a certain amount you won't be able to push the apply button anymore, because it's hidden under your taskbar (assuming it is at the bottom) or, if you don't have a taskbar or it is set to hidden mode, it will be out of visible area.

This is very problematic, especially, if you want to change your resolution.

Reproducible: Always

Steps to Reproduce:
1. Open display config dialogue
2. Drastically decrease your resolution
3. Try to increase resolution again; Good luck finding/hitting the apply button (alt + a will do, though)

Actual Results:  
I was not able to hit the apply button with my cursor.

Expected Results:  
Even though a window has a certain minimal size, it should always (!) be possible to click all elements of a window.