My problem is that AltGr just randomly and frequently stops working. I've only seen this problem since the last update (10.1). The only sure way to recover functionality that I've found was to change the keyboard setup (alt+shift). I'm using multiple keyboards on my system so I need to rotate between the languages back to the one I use in Mathematica every time this happens.
I saw Is it possible to get AltGr key to work in Math.exe? but it seems to be on another issue.
I'm running Windows 7.
Considering how I can recover the functionality, I suppose that it might be a Windows issue, but I've not noticed this behavior anywhere else.
EDIT: I just noticed that when AltGr looses it's 'normal' functionality, it activates the menu bar instead.