This is pretty minor compared to many of the other comments, but my dad has one habit that drives me mad.
If my parents and I are having any kind of discussion where not everyone's opinions match, be it friendly debate or big blowout argument, if my opinion on the matter even remotely resembles my mother's, I'm "only saying that to agree with her". And nothing will convince him otherwise. The awkward thing is, as I'm very similar to my mother, yeah, it's often likely we'll have a similar thought process and opinions.
Funnily enough, if I agree with him on something, he never accuses me of just saying it to agree with him!