Rails 3 Routing Error after Redeploy

I'm hosting a Rails 3 site on Heroku, and I've found that whenever I push a new release, POST requests raise a RoutingError until the user clears his cookies. I don't have any problems while developing locally; just when I push to Heroku. So you can hit the front page, but you get a RoutingError when you submit the Sign Up and Log In forms. Has anyone else seen this behavior? Any ideas how to fix it?

I really don't think the problem here is my config/routes.rb or in my controllers/views, because normally this works, but it breaks when I push a new release, at least until cookies get cleared. Could this be a protect_from_forgery issue?

If it helps, here are my logs:

2011-08-29T16:42:30+00:00 heroku[router]: POST mysite.com/signup dyno=web.1 queue=0 wait=0ms service=354ms status=200 bytes=7518
2011-08-29T16:42:30+00:00 heroku[nginx]: 128.91.71.89 - staging [29/Aug/2011开发者_StackOverflow:09:42:30 -0700] "POST /signup HTTP/1.1" 200 2482 "http://mysite.com/" "Mozilla/5.0 (X11; Linux i686; rv:6.0) Gecko/20100101 Firefox/6.0" mysite.com
2011-08-29T16:43:17+00:00 app[web.1]: 
2011-08-29T16:43:17+00:00 app[web.1]: 
2011-08-29T16:43:17+00:00 app[web.1]: Started POST "/signup" for 128.91.71.74 at Mon Aug 29 09:43:17 -0700 2011
2011-08-29T16:43:17+00:00 app[web.1]: 
2011-08-29T16:43:17+00:00 app[web.1]: ActionController::RoutingError (No route matches "/signup"):

Well, it turns out the problem was the login & signup forms were doing a PUT, not a POST, because @user was wrongly getting set under certain weird conditions. Sorry to waste the time of anyone who bothers reading this!