I remember Michael Palin remarking that they'd got it wrong when making Monty Python and the Holy Grail - they'd blacked out people's teeth, assuming that in the absence of dentists, teeth would rot and fall out. Of course, without sugar, medieval teeth were in better shape than our own, as can be seen in surviving skeletons of that period.
Another misconception is that no one washed in the Middle Ages. In fact, bath houses were popular, and some castles had running hot and cold water, and a full time bathman.
One of the most charming rooms in the twelfth century part of Leeds Castle is the Queen's Bathroom, with its draped barrel-shaped bathtub, with a tent-like canopy to keep off the draughts and afford a little privacy. Hot water, with herbs and flower petals, and a maid to hand you a towel - real luxury.