Generally a graffito stating "Fuck Penders", in reference to comic writer Ken Penders, a former writer and artist for the Sonic the Hedgehog comics.

Some history: In 2009, Penders began copyrighting everything he made for the comics, which brought him in conflict with SEGA (Sonic's owners) and Archie Comics (the company producing the Sonic comics under SEGA's licence). In 2010, Archie sued Penders in an effort to assert ownership of the characters he made; the suit was settled in 2012 after Archie replaced its legal team, resulting in Penders being dismissed and Archie only being allowed to use characters he made if he reviewed and approved the stories they were being used in (in essence, requiring Archie to make him a second licensor). As a result, Archie and SEGA wrote out all of the characters Penders had created and (in the Worlds Collide Sonic/Mega Man crossover comic) retconned out every storyline that involved them. Fans largely blame Penders for the situation; hence the graffito.