A top-three Skinner P.A. gag. Other two:

During the Radioactive Man episode when he calls him Radio Man and Nelson says “It’s Radioactive Man, stupid!” and Skinner says, “Strange, I shouldn’t have been able to hear that.”

And the other one is during the Valentine’s Day “Choo-Choo Choose You” episode where he’s talking about Bart’s candy hearts and has a PTSD Vietnam flashback over the PA.