[Twisted-Python] Strange recursion error with twisted.web

[Peter Westlake]

I'm getting a "maximum recursion depth exceeded" error that appears to be coming from flatten(). The odd thing is that it only happens sometimes. The HTML that's being flattened does have a few Deferreds in it. Those come from function calls, which cache the results, which might explain why I only see the error on the first visit to the page (as far as I can tell). 

The system recursion limit is the standard 1000. My HTML is only nested a few tags deep, two orders of magnitude short of that. Is there anything about the way flatten() works that might cause this behaviour?

Peter.