HTTP response was too large: 3410251. The limit is: 1048576.
(The number 3410251 above might vary completely arbitrarily)
Solutions like this won't help you, because you know for a fact that you have no file that large or anything that could reasonably cause it, then it might be because you have a maximum recursion depth reached error. This causes an enourmous stack trace which will exceed Google App Engines maxiumum response length (1MB). Try sweeping your code for any place where infinite recursion might occur.