It's not a huge difference. Your beer will be slightly dryer, but will certainly be drinkable.
I'd bet as many as half of the AG brewers out there have inaccurate thermometers. You at least know your thermometer is bad, that puts you ahead of them.
My preboil is 1.032...... I am hosing this up so bad..... fist ag by myself. And on top of it I am doing a double brew. And can't get second mash above 142... grrrrrr!!!