That's a habit from teaching French, where there is almost (see, I can't help myself!) always an exception.
Heh. That reminds me, yesterday, I was talking to one of my classmates about our foreign language exams. We've each got to be able to take a few pages of a math paper written in either French, German, or Russian, and, with the aid of a dictionary, translate it into English. I took French every year from seventh grade through sophomore year of college, so I'm not too worried. My friend doesn't know any of the required languages, but she figures she can wing her way though French. I told her that, since she's got a dictionary, the only things she really needs to know are conjugations of etre, avoir, aller, and faire, plus be able to recognise the conditional verb tenses. She was giving me such strange looks when I showed her some of those conjugations.