Do you have a second language/keyboard installed on your system? Then
it could happen that you're switching keyboard layouts, e.g., when
incidentaly pressing left-Shift-Alt I can switch between english and
german keyboard (and on german keyboards y and z change places).
Is there a way to disable this shortcut to prevent it from happening again?
If it's the language selection then double-click (I think it's a double-click) on the language switch in the System Tray: the Keyboard Properties dialog will appear (you can find the same dialog under Windows's menu Start>Settings>Keyboard). Select the Language tab and set the Switch Languages to None.