I would say that not accounting for a browser bug is definitely their fault. Remember that they have a responsibility to their users to make their site work by

I would say that not accounting for a browser bug is definitely their fault. Remember that they have a responsibility to their users to make their site work by accounting for bugs. Designers spend a lot of times meticulously working on CSS to account for all of the various browser rendering quirks. Developers need to put the same care into their JavaScript. Perhaps Twitter has chosen not to support Opera which is fine by me (even though I'm a user), but I think more than likely this was a slip-up in their QA process.