Evan, it appears in the web interface of identi.ca, I just reproduced the error. I was logged in identica, so I logged out. Then I went to https://identi.ca/aeva, and it appears the "Follow" button. I click there, then it redirects me to the login page, I login, then it redirects me to https://identi.ca/aeva and... the follow button is there! (although I'm following her). I press the Follow button, and I got the error. After getting the error, I update the webpage (go to the browser address bar and press enter), and the "Follow" button dissapears (the error too), and the "Stop following" button appears. Maybe something related to closing the session, browser cache or things like that?