Conversation so far:
What activities did the Round Table do?
The group was devoted to games, including cribbage and poker.
Did they play a lot of games?
The group had its own poker club, the Thanatopsis Literary and Inside Straight Club.
What else did they do?
In addition to the daily luncheons, members of the Round Table worked and associated with each other almost constantly.
Did they work in writing?
CANNOTANSWER.
What did they do for work?
silk merchant Paul Hyde Bonner, baking heir Raoul Fleischmann, actor Harpo Marx, and writer Ring Lardner.
What else did the club do?
A number of Round Tablers were inveterate practical jokers, constantly pulling pranks on one another.
Did they pull any good pranks?
Harold Ross and Jane Grant once spent weeks playing a particularly memorable joke on Woollcott involving a prized portrait of himself.
what was the prank?
They had several copies made, each slightly more askew than the last, and would periodically secretly swap them out.

Question: What happened when he noticed?
Self-contained rewrite: What happened when Woollcott noticed the prank that was played on him by Harold Ross and Jane Grant?