As there currently isn't any method of tagging docks that are unable to dock, I sometimes repeatedly try to lock my bike to the same dock every night like a fool, as I don't know if it has been fixed yet or even which dock it was.
So I made stickers as a memo for myself! Which will in effect save lots of time and frustration for other people from slamming their bikes at broken docks - they can just see the memo and try a different one. This also might save the Citibike team time while they search for the right docks to repair. Totally suggesting crowdsourcing an error report by making stickers available to those samaritan's who would take an extra 10 seconds to tag a dock right after noticing it isn't working. Bulk stickers are cheap and easy to carry. And so this seems like a good intermediary solution till there's an error button, similar to the one the bikes themselves have.
Let me know if this is helpful! (or if I'm being meddlesome and should stop doing this).