From your garden variety troll to far worse kinds of harassment, we've all run into people on the internet that we'd rather not deal with. On Twitter, choosing to block someone used to mean they couldn't see your feed while they were logged in. Now, a support page explains that it just means you won't see their tweets or interactions with you, unless you go to their page or they come up in a search. The only way block actually blocks someone, and forces them to unfollow your account, is if your tweets are set to private.
Many users are voicing disagreement with the change, citing the fact that harassers can easily monitor their target's accounts, or RT their tweets and encourage others to join in. As it stands, the harassed now have to choose between participating in public conversations or controlling who can follow and retweet them (temporarily switching to private and blocking someone can be used to cause a force-unfollow in a pinch).