Demi Lovato -- How Dare 'X Factor' Joke About My Drinking Problem

Demi Lovato is furious at the producers of "X Factor," grousing that she was blindsided by a skit that mocked her addiction ... TMZ has learned.

Mario Lopez introduced a skit which showed clips of Demi drinking what looked like booze, acting annoying and off center.

Demi is extremely upset because addiction has turned her life upside down. It got so bad, she was living in a sober house for more than a year just to avoid temptation.

You can see her reaction on the show ... at one point it appears she says "That's f**ked up."

Demi's already said she's leaving the show, but now it's pretty clear ... for her it couldn't be soon enough.

We contacted reps for the show ... so far no word back.