What causes this?

At times when using audacity, I seem to inadvertently press a hotkey which causes the audio to become insanely loud, and sound muffled at the same time. It’s very annoying. I have a keyboard with the typical qwerty layout. This key seems to be on the bottom row, but I can’t nail which one it is. I’d just like to trying and prevent this from occurring if possible, because it’s almost like an explosion in my ears.

Does anyone know what could be causing this?

Thanks in advance.

Using Windows 7 Home Premium, Audacity 2.1.0

What do you do to fix the problem when it occurs?
If you stop playback and then restart playback, is it still “insanely loud” or has it magically fixed itself?
Have you customised any of the keyboard shortcuts?

By the way, the current version of Audacity is 2.1.2 which is available here: http://www.audacityteam.org/download/windows/