When attempting to use en.m.wikipedia.org on a desktop browser (tested w/ Chrome 28 and FF 22), I get a red warning message after logging in:
"Central user log in
No active login attempt is in progress for your session."
This message is displayed via the desktop site and so I have to manually return to mobile mode. It does log me in properly.
This is without beta or dragon mode enabled.
I can't reproduce this behavior on a mobile device, but in addition to causing issues with testing, it might affect tablet users who want to use the mobile site.