The CTA has had at least one recent incident of a person stealing a bus and impersonating a driver. That was in September 2010.
But that guy is a piker compared to "transit super-thief" Darius McCollum. He has been arrested 27 times in 30 years for illegally commandeering buses and subway trains.
And apparently, bus thievery is not an uncommon occurrence. The Atlantic Cities has compiled 10 notorious cases of bus thieves from around the nation and their thrilling escapades. It's worth the read.
But please don't get any ideas about stealing my bus.