Oh yeah, you let it slip and people think they're right. Usually what I do goes something like this:
Innocent (enough) Mis-namer: "Hey [birthname] can you help me on this?"
Me: "Hey Andrew (with slight emphasis) can you help me on this?"
They usually blush, stumble, and/or correct themselves and apologise. Luckily I only have to suffer through it in a classroom environment until the end of the year, then I'll be starting fresh at college. In any case, I wouldn't feel too bad about correcting people; it seems that when you don't at least once or twice, they won't bother to correct themselves, and you'll end up being misgendered and called the wrong name a whole lot longer than if you just give them an occasional nudge in the right direction.