"give the impression that twitter is somehow at fault"... which it is. This is definitely partly Twitter's fault for craming all of their code as one humongous instruction (comma) rather than multiple lines (semi-colon.) It's also obviously partly Opera's, for not handling terrible code such as Twitter's as well as they should have.
"Terrible code" is this sort've nebulous subjective thing. Clearly, to you and I, this is terrible code, though probably created by a minifier / compiler.
Lots of code can be terrible (We've all written some in our time). However, if terrible code is valid it's the interpreter's (in this case) problem to figure it out. The ECMAScript spec is nightmarish but we get the spec we deserve :). Twitter has ZERO fault here. They wrote terrible valid code and the parser / interpreter should be spec conformant no matter the pain.
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.
In this case both the current title and the original title (if memory serves) give the impression that twitter is somehow at fault.