I'm always interested in hearing people's suggestions and learning about pain points that I haven't experienced myself.
Here's one (I've waited a few hours before posting to trigger it again, but I'll only know if it was long enough when I click Post):
Your session timed out while posting. Please try to re-submit your message.
It's probably not a bug but a feature, but I've seen this far too many times. I often leave a tab open and come back to it hours later.
Sometimes it takes me days, in that case I don't mind the error. A couple more hours would be appreciated.