I found B Gata H Kei a pretty boring, mediocre series and never finished it, but apparently some people got very worked up about it: Anime News Network reported in 2010 that the studio and writers responsible for it received "multiple threatening letters" demanding "changes in the anime's content and presentation".
Those who have watched the show know that it really has very little sexual content and is overall very tame; like its protagonist, it's all talk and no action. This makes it a curious target for such passion. The comments on the ANN story have a lot of speculation, and this Reddit thread posits the senders were angry over the dirty things being spoken by Yukari Tamura, but I'm curious if any details were ever released. Do we know what kinds of changes these letters were demanding, and what it was in the show that made people angry enough to threaten its creators with physical harm?