All the local buses in Bristol used to have (and may still do) "Welcome to FirstBus" vinyls on the window adjacent to the front door. I saw one vehicle several years ago that had had some of the stickers peeled off the reveal the message "We come First".
There have also been some "sign hacking" on trains round here recently... All US public transport vehicles seem to have a "Title VI notice to the public", which essentially promises access to vehicles and seating without discrimination on the grounds of race/national origin etc - the text of the notice on SEPTA, my local system, is here:
http://www.septa.com/titlevi.htmlSome cheeky individual has been posting very official-looking altered notices with rather different text (slightly tricky to read in this photo, but worth the effort - some of it's local in-jokes but a lot of it's not):
