What make/model is it? Does any hardware look distorted?
Does it wobble in directions that it shouldn't (side-to-side)?
My first thought would be that the hinge bolt is loose. Take care working around it, though. It's spring-loaded.

Another possible cause is that the fence is out of alignment. Usually there's a way to adjust this.

miter saw alignment

Randy, I'd have to do some gymnastics to get at it right now to give you answers. I don't remember it wobbling, but I will check all the bolts on it. In terms of years it's not young, but in terms of usage it sure is.

Bobsmyuncle, thanks for that link! I've book marked it. I'm going to be hauling that saw out again within a few weeks for a job I want to do, and I'll consult with that and see if I can fix it.

Thank you both!

Checking with a square or triangle is good, but I prefer to grab a scrap that has parallel sides and make some test cuts. Generally this will double the amount of error when you put the cut edges together and allows you to get spot on.