Very often it's just trivial stuff like not knowing the proper titles and forms of address -- it doesn't alter the plot, but causes me to lose faith in the writer so that I'm not willing to believe anything that might come across as at all doubtful. (Whereas if the author has earned my trust, I'll swallow much greater improbabilities whole, thinking the author must have dug up some information I wasn't aware of.) Other times it's just lost opportunities, where the author misses legitimate chances to ramp up the tension and chooses to instill the necessary drama by having the characters act stupid instead. I'm sure you've noticed how much romance novels (and other books, too, but it's a staple of romance and soap opera) depend on characters not speaking up when they should and would, so that misunderstandings may flourish and lovers can be parted for years at a time. If acting dense were the only way to create these interesting situations that would be one thing (if the lovers settle down together in chapter five, what do they do for the rest of the book?), but when more plausible ways have been by-passed because the author doesn't know better, then I get annoyed.
Or another example: the hero (or heroine) mistakes the heroine's (or hero's) identity on the basis of mistaking her/him for a peasant or a menial (and much confusion ensues), when the exchange of a few words, or a glance at the hands, would immediately tell them otherwise. In most times and places, it would be very hard indeed to pass yourself off as belonging to a different class, much less being able to do so on the spur of the moment when someone has pissed you off by mistaking you for something you're not. That's why it was so hard for people to alter their positions in life, because class distinctions could mark you so indelibly that you always carried your origins around with you.
