Hands down, the biggest complaint I get is that Microsoft Word seems to have a mind of its own when it comes to formatting. People swear they did nothing more than breathe on their document, and things went completely wonky!
Of course, without actually standing over their shoulder and watching them work, it's really impossible for me to know <em>exactly</em> what happened. A lot of times, there's a pretty easy File > Options tweak that could prevent similar snafus from happening again. (And don't even get me started about why you need to learn to use Styles.)
But in my experience, most people aren't particularly interested in trying to figure out how it happened. They just want to fix it and move on.
So for that crowd, I've put together a two-minute video on the four fastest ways I know to basically nuke your formatting so you can start over. You can basically choose among these: