I know someone who baked their headlight at 350 degrees for 20 minutes...wasn't pretty (and wasn't me).
I was working on their car and he said, "Oh sh1t! My headlight is still in the oven." The housing looked like a gooey chocolate chip cookie. Luckily, it still went in and everything worked. Although the ballasts really didn't go in too well.
I personally haven't had any issues. I like my DDEs.