They make changes without using revision marks, so you have to pore through the documents to see what they've messed up. And they put comments directly into the text rather than using Word's Comments feature. Their spelling may leave a lot to be desired, too but perhaps not as much as their grammar, which they may allow Word's grammar checker to "improve" with its suggestions.
As you probably know, Word offers a slew of features to help keep your colleagues under control and fix problems with documents they've edited but these features bring their own annoyances, too. This tutorial walks you through annoyances brought on by both your colleagues and Word itself. You can often kill two annoyances with one stone double the satisfaction in half the time. We'll start with a quick look at Word forms, then take a look at two features most of us love to hate: Track Changes and the spelling and grammar checkers.