If you ride the T in Boston then you know about Charlie, the guy on your fare ticket/card, but do you REALLY know Charlie? So Charlie isn't a real guy and he popped up on the fare cards in 2006 when they were trying to replace coin tokens. Charlie is a tribute to the song "Charlie on the MTA," which tells the story of a young man named Charlie who couldn't afford to pay the nickel for his exit fair and thus was stuck on the subway forever. Though the song also talks about his loving wife who comes to bring him food everyday. The Massachusetts Bay Transit Authority used to be called the Metropolitan Transit Authority.
So always have change as this can surely happen to you! Actually it isn't possible on the T since you don't have to pay an exit fair. You can even buy a day, week, or monthly pass and ride the train and bus as often as you like :) I highly recommend it! I once took a trip to the end of each line and here are pictures from my trip: