The problem is that even though you have a monthly pass, cause you couldn't produce one when asked, then they have no proof. Also, not that I'm saying you did, but if I was an officer, you told me you forgot your pass, you could have actually lent it to a friend that day which is why you didn't have it on you. There's a lot of variables, and I'm sure they've all heard the same excuse before. I would just say if you can't afford the ticket, dispute it. Keep your this month's monthly pass so you can bring it to court. If you also had the receipt from when you purchased it, that would be good too cause you can show you had bought it before that day. Just plead guilty and tell them your story and hopefully you can get a fine reduction.