Apparently the monthly pass policy has been that you must be in the system for 20 minutes before you are allowed back in once you exit. This is to discourage "pass-backs" of monthly passes. That's fine, though 20 minutes is too long. So with the old turnstyles, you'd be locked out, but could always flash your pass at the person the booth, and they'd let you in the handicap entrance.
Not so with the Charlie Card. There is still the 20 minute window, but now they won't even let you through, as experienced by the guy in this article:
Well today it happened to me.
Today I had to get out at South Station to get some cash, as there are no BofA ATM's in the Marine Industrial Park.
So I got to SS and politely asked the CSA - who was a complete bitch, literally, I was 6" from her saying "excuse me, excuse me" and she just kept looking the other way - if I could come back in, as I had a monthly pass.
"Have you been on for 20 minutes?"
"I think so, but I'm not sure"
"Then you'll have to pay again"
"But I've got a monthly pass, you are seeing me exit, and I will come to you to get back in, can I do that?"
"I don't care what you do, as long as it's been 20 minutes."
Former token sellers are now referred to as CSA's, or Customer Service Agents. I'm thinking that the acronym really stands for inCompetent Surly Assholes.