Null reference exception with a twist.
One Robert Barbour from Los Angeles was fond of sailing. He tried to get a custom license plate for his car. His choices were 'Boating' and 'Sailing.' The official form contained three choices and all were mandatory. So, as his third choice, he wrote 'No Plate.' He was allotted 'No Plate' as his license. Surprisingly, he started receiving 2,000 tickets per month because officers marked illegally parked vehicles without license plates as 'No Plate' and raised void tickets. He even had to appear in court a few times to explain the situation.
The placeholders and markers in one part of the system may become valid data for another.
Other interesting custom active licenses in California are:
XXXXXXX (Received $19,000 of parking tickets)
Indeed, very little choice for the cops