I've been using HTTP Toolkit for a few months. It's worked great for viewing traffic in a game from the app store (I've modded the app to view it's traffic). I wanted to try using mitmproxy (rather, mitmweb), but it was going to require its own cert to be installed. I knew that HTTP Toolkit copies the cert folder and remounts, so I thought I'd follow the steps mitmproxy provides to make the certs (http://wiki.cacert.org/FAQ/ImportRootCert#Android_Phones_.26_Tablets) and then connecting with HTTP Toolkit would do it's thing and the mitmproxy certs would get included.
It didn't work.
Now I don't recall why I first rebooted my phone afterwards, but I haven't been able to log in to the app while HTTP Proxy is running since this happened. It used to list everything and just have a line that says "Certificate rejected for ...." but now the app refuses to login with Google Play; even if I have only the target app selected.
Things I've tried: I've disabled and uninstalled the cert from HTTP Toolkit so it could re-install it. I think the mitmproxy certs I created ended up not even being in the folder after I started HTTP Toolkit. I've uninstalled the HTTP Toolkit app. Rebooted my phone a couple times. Nothing appears to have changed with the app; it's still the same, latest version.