Comment 4 for bug 631262

The reported error is given when a request sits in the ha proxy queue
for more than 30 seconds without a response, or if a forwarded request
isn't answered by the appserver.

Its the latter case I think is happening:
haproxy forwards a request to the server
its either being acted on or just starting/queued in the appserver somewhere
ctrl-C is hit
the request is dropped on the floor, socket closed
haproxy sees the error and reports it to the user.