It's called an "excursion fare", which is meant for those that just ride the train without getting off and come back to the same station. You can talk to a BART station agent (assuming you can find one) and they'll let you out, or call customer service and they'll reverse the charge.
Modern fare systems should be able to figure out when you've exited right after entering and not charge you. BART is supposed to be adding a 30 minute grace period so if you go in and out of the station within 30 minutes, you won't be charged.
