You're most at risk with something which you loved when you were at a different stage in your reading life, I suspect. I love the Golden Age detective writers - Sayers, Christie, Allingham - and used to love Ngaio Marsh. Most of them have at least a whiff of the anti-semitism and other racisms of their time, and of course plenty of sexism. But Marsh also has a really nasty homophobic streak (surprising, if we're talking stereotypes, since her main career was in the theatre) which makes some of her books completely unreadable now.
More seriously, if you like, I adored the children's writer Antonia Forest well into my teens, and I went back to her as an adult. She writes extraordinarily well, with a subtlety and sophistication about human relationships which is very unusual. There
is a conservative strand in the books, which as a child I'd logged and moved on from. As an adult I also just thought, 'Well, she was born in 1915 and this was written in 1956, so what do you expect?' and moved on. And then the republishings had forewords from her, and all her deep-dyed conservative reasons for writing what she had suddenly became part of her agenda. I still think they're extraordinarily good, but in a technical way. It's a bit melodramatic to say I feel betrayed, but...
I still count myself a fan, though, and did a piece on her for Vulpes Libris:
http://vulpeslibris.wordpress.com/2008/07/10/celebrating-antonia-forest-the-grown-up-childrens-author/
Emma