I've had that drummed into me since I was young. I don't know who first talked about it but it has been a pet peeve of my older sister's for years. It grates a little for me, but I just think it is interesting. So many say, for example, "That skill is my fort tay" thinking they are meaning "a strong point" when, in fact, they are saying that skill is my "loud". I think it has to do with wanting to add that Frenchified "tay" to the end of word ending in e.
Of course, when I say something is my forte... which I rarely do, not being particularly good at things and a good reason to be humble... I carefully pronounce it. Probably most people think I'm wrong. That's painful.
Here's what the American Heritage Dictionary online has to say about it:
The word forte, coming from French fort, should properly be pronounced with one syllable, like the English word fort. Common usage, however, prefers the two-syllable pronunciation, (fôrt), which has been influenced possibly by the music term forte borrowed from Italian. In a recent survey a strong majority of the Usage Panel, 74 percent, preferred the two-syllable pronunciation. The result is a delicate situation; speakers who are aware of the origin of the word may wish to continue to pronounce it as one syllable but at an increasing risk of puzzling their listeners.