In Pennsylvania, the Pittsburgh Steeler American football team is the official licensee of the Terrible Towel trademark (see link).
In Indiana, someone decided to make a Terrible Mask (see link).
As you can expect, the trademark holder sent a cease & desist letter to the seller. The seller however did not back down. Instead, the seller alleged the existence of a non-interference / co-existence agreement with the trademark owner in 1975. As you can expect, a lawsuit was filed.
Interestingly, when you go to the Terrible Mask website, you are greeted by a pop-up seeking customer testimonials about a Terrible Shirt they have been selling since 1975.
Photo by Appleseed15219 on Wikimedia Commons