The victim: an unsuspecting Kristen Stewart just trying to live her life.
The prank: someone wrote "I Love Rob" on her car.
Analyzing the prank: the Rob from the prank is referring to Kristen's ex Robert Pattinson. Someone thought this was a funny joke because they are broken up.
The aftermath: victim drives away, doesn't seem amused by prank.
The lesson: Kristen Stewart needs to wash her car and also; be careful who you prank.