While filming the Seinfeld episode "The Parking Garage", Michael Richards (Cosmo Kramer) cut his lip when dropping an 40kg air conditioner into the trunk of a car and tries to stay in character despite his injury.

Michael Richards used real props like a real concrete bag in the revenge episode

https://m.youtube.com/watch?v=JdOwTN4-n1I&pp=0gcJCdgAo7VqN5tD