What to do: Check Gmail. You're probably logged in under another account.
Error type: An Alert about an internal error
What to do: Don't worry. This seems to be a problem on G's end. Click the Acknowledge button and return to home to continue working. If you get the error several times in a row, take a break and report the problem using the email listed in the hub in the "Reporting a Problem" section.
Error type: Feedback submission failed
What to do: Check your history. It's most likely a glitch and your tasks actually are being submitted.