Hum ... tough one. Mainly because if it's that bad I get rid of it as fast as possible. But I know I've read a few. Most recently I was disappointed with Todd McCaffrey (Anne's son - has taken over writing about Pern). I read something he wrote with his mother and quite enjoyed it but then bought Dragon ... erm ... heart? (can't quite recall now) which he wrote on his own. Several uncomfortable moments and quite a lot of bad writing.
