Okay I confess, I still use Word and Excel 2003 (not sure if this applies to the 2007 versions). Here's the bugaboo that drives me crazy most frequently, especially when I'm writing in Word.
Overtype Mode (OVR).
According to Microsoft's web site, OVR is only activated when you hit insert or INS in Word and in Excel, it requires double-clicking on a cell or hitting F2.
I don't believe it; at least not in Word.
I find that OVR strikes quite arbirarily. For those of you who don't know what I'm talking about, its that weird quirk Word has to randomly start eating your words when you backspace or click in mid-sentence to add copy.
The way to stop it is to click on the OVR box in the lower tool bar until it goes pale.
Let me just add, I am a professional writer by trade and I've never used OVR as a tool. It' only serves as a nuisance.