Goodbye (1) - S2-E4
Corrected entry: In the show, Paul Henessy dies in what is presumably a car crash (Kerry says something along the lines of, "If he had just seen the batteries in the drawer." in this episode), after he went to the supermarket. This episode followed the death of John Ritter, who played Paul. John collapsed on set and died on September 11, 2003 from an aortic dissection (heart condition) that went undetected until he died.
Correction: Actually Paul presumably dies of the same condition as John Ritter. There are mentionings of him passing out at the supermarket and never wakening.