As with all From Software games, Bloodborne, for how good it is, is also a fairly buggy product. Most of these are harmless, at most a little annoying and frustrating in gameplay, maybe a little hindering to your overall progress at worst. However, there is also a game breaking bug in the game that Sony has just detailed (so you know how to avoid it), that they have now announced that they are working to fix.
Essentially (keeping this as spoiler free as possible), this has to do with trying to help someone do the Forbidden Woods (a section in the game that appears a fair bit in) as a guest in co-op, or just trying to hinder them as a hostile invader. In both these cases, a very important item needed to progress will fail to appear in your game after you do this.
Sony and From Software are working on releasing an update for this issue immediately- it will be patch 1.02. it will not fix the game’s infamous stutter or load times, however.
For now? I recommend just not doing co-op when you get to the Forbidden Woods at all.