Pressing backspace in Microsoft Word doesn't delete the text

This has happened to me a couple of times, and I don't know how, but I got into a state where when in Word, when I selected text and pressed the backspace key, it would move the cursor to the front of the word instead of deleting the text. The delete key still worked.

To fix this, check this box: Tools | Options | Edit tab | Typing Replaces Selection.