"The Chair of Death" is Sheldon's name for a chair that Penny found in the street and brought into her apartment in the episode "The Infestation Hypothesis".
Before Sheldon learns where the chair was from, he states that it is comfortable because it supports the
Amy gets bit when she sits in the chair
lumbar region and "cradles the buttocks". When Penny tells him that it was an abandoned chair she found in the street, Sheldon immediately assumes that it's unsanitary and strongly urges Penny to get rid of the chair, which she refuses. Four times Sheldon knocks on Penny's door with various scenarios having to do with health and decor for her to get rid of it and she refuses every time finally scaring Sheldon away with the cushion from the chair. After a nightmare that involves him sitting in the chair again only to be covered with bugs, Sheldon insists that Amy try to convince Penny to get rid of the chair.
Amy tries to get Penny to get rid of it due to Sheldon's insistence, until Penny saw through his plan and told Amy to leave. Amy immediately sat in it to appease Penny; however something inside the chair bites Amy in the butt, assumed to be a rodent of some sort, causing Penny and Amy to run out of the apartment screaming.
At the end of the episode, it is discovered that Penny abandoned the chair in the front of the apartment building because it is found by Howard and Raj. They decide to move it up to Sheldon and Leonard's Apartment, so that Raj wouldn't have to sit on the floor anymore.
The animal that bit Amy can still be seen as Raj and Howard carry the chair back into the apartment building.