Send In Background Error

I noticed a funny behavior in my Gmail for the last week or two and finally this morning tracked it down.

In the middle of a session, I was seeing the terrible "Oops...the system encountered a problem (#707)" message pop up, along with the exponential backoff for the retry:



Unfortunately, it doesn't snap itself back, and clicking "Retry Now" doesn't work either.

The only way to deal with this situation is to close the tab. A further clue is that on closing the tab, Gmail warns us that we should leave yet... it's trying to do something.




Turns out this has something to do with the "Send in Background" labs feature.



I can consistently reproduce the error by:

1. Open Gmail with "Send In Background" enabled
2. Open any message in my inbox
3. Type a reply
4. Hit tab-enter to send
5. Immediately hit "y" to archive
6. Error
   

Turning the "Send In Background" feature off and walking through those steps again produces no error.