My favourite bug (on Android) is when you paste a URL, hit enter to send, then start typing your message related to the URL. Because it wants to process the URL, look it up for a preview, it doesn't send straight away. However when it does, it also includes your absolutely not submitted and presently in draft stage beginnings of your next message.
Unreal.
Shoulda just stuck with Skype or something else.
They removed a button from hangouts a while back that let you send your location to the channel. I guess it wasn't used much, but it was useful to me. Made me miss protocols and RFCs.
The closed captioning in Chat is pretty amazing, though.
You'd think they designed it like that because it works well for their paid use case, using it for work. But no, it's really annoying to use in that context to. I can't call a colleague, instead I have to add an impromptu meeting to the chat, join it myself, wait until they show up. I mean, it's just two of three clicks instead of one, but still.