What's Your Best Externality?

The last two years I’ve run an “externality” contest in my giant intro class, offering $5 to the student who comes up with the best example.
Both positive and negative externalities are welcome. I call on five students to tell the class their examples. My favorite this year was the student whose brother started wearing cut-off t-shirts (muscle shirts) but promptly proceeded to gain 100 pounds of fat. Worse still, the family name being Quackenbush, the brother had a duck tattooed on his flabby upper arm.
By voice vote, however, the class awarded the $5 to a woman who offered the negative externality of the throbbing bass played full-blast on the pickup truck parked in the lane next to you at a stoplight.