Chargers general manager Tom Telesco was on the bus, along with a group of San Diego players.
According to a team spokesperson, players were surprised by the incident, but otherwise laughed it off.
San Diego retweeted a picture of the mess.
The Chargers and the Raiders have been longtime AFC West division rivals. There's always an extra edge between the two teams and fan bases that comes with the two teams facing off.
Sunday's contest will be Oakland interim coach Tony Sparano's first chance to lead his team since the Raiders fired former coach Dennis Allen two weeks ago.