I was reminded of this today by a note from a training client (thanks Laurie!): don't get the Return and Enter keys on the keyboard mixed up. They are not interchangeable in InDesign and QuarkXPress. Here's why:
When you are typing text in either program, the Return key starts a new paragraph. Hitting the Enter key (on the numeric keypad) inserts a Column Break in InDesign and QuarkXPress, which forces the text that follows to jump to the top of the next column in a multi-column text box, or to flow out of the box if it is a single-column text box.
So if you accidentally, or out of habit, press the Enter key when you should press the Return key, your text may "disappear", jumping to an unexpected location.