Thunderbird popped this one up one day:

I cannot remember the context, but then I shouldn’t have to! So, basically some unspecified operation failed, for an unknown reason. It’s nice they mentioned the network connection didn’t get cleaned up, though it’s rather useless information since I don’t know what should be done about it, let alone what the impact is. As a programmer I can guess that the dangling network connection is just a minor adminstrative detail which will get cleaned up on the next reboot. But I can imagine my mother getting this error message and being worried that viruses or spammers are going to sneak onto her computer this way.