As exarkun already noted here: <http://twistedmatrix.com/trac/ticket/5028#comment:2>, this is caused by foolscap manipulating an attribute which probably shouldn't have been public in the first place, and *definitely* isn't mutable.  This means that there are several other types of instrumentation which foolscap will break.

This is a popular ticket this week - I just mentioned it in another thread: <http://twistedmatrix.com/trac/ticket/3204>.  If foolscap needs to switch protocols mid-stream, it either needs to do what AMP does, or contribute a real fix for that ticket.  (And then require a new version of Twisted :).)