According to Suetonius, Ptolemy of Mauretania (probably the richest of the vassal kingdoms of Rome at the time) visited Rome to swear fealty to Caligula, and while on a diplomatic tour, they went to the theater, were the public were more impressed with his purple tunic than Caligula's. So of course the little psycho had him murdered.
(Other less fun historians disagree on the reasons - but everyone pretty much agrees that Little Booties had him murdered)
GW