A theatre joke is:
 
”“Did you hear about the actor who fell through the floorboards? He was just going through a stage.”
 
The “just going through a stage” pun has been cited in print since at least 1993.
 
     
20 April 1993, Chicago (IL) Tribune, “Bonus jokes of the week,” sec. 7, pg. 11, col. 5: 
Q. Did you hear about the actor who fell through the floor of the theater?
A. It was just a stage he was going through.
 
Google Groups: rec.arts.theatre.musicals
CLIP ART
EDWARD BAYE
8/17/99
ART!!!!!!
Did you hear about the actor who fell through the floor??
(...)
Don’t worry it was just a stage he was going through!
 
Google Books
Movie Madness
By Julia DeVillers
New Albany, OH: Too Brands Investments
2004
Pg. ?:
“Did you hear about the actor who fell through the floor?” Kacey asked. “It was just a stage he was going through. Mwah-ha-ha!”
       
8 January 2005, Newsday (Long Island, NY), Kidsday sec., pg. C18:
Did you hear about the actor who fell through the floor?
It was just a stage he was going through.
Nicholas Krebs
 
Google Books
Penguin Pocket Jokes
By David Pickering
London, UK: Penguin Books
2006
Pg. ?:
Did you hear about the actor who fell through the floor? It was just a stage he was going through.
 
Twitter
Matt Saraceni‏
@iamnotmatt
did you hear about the actor who fell through the floor?
he was just going through a stage.
haaaaaaaaaaaa
10:13 AM - 26 Jan 2009
 
Twitter
❤️ FizzyDuck ❤️‏
@FizzyDuck
Did you hear about the actor who broke through the floorboards? He was just going through a stage.
11:43 AM - 9 Jul 2009
         
Google Books
1001 Kid’s Jokes
By Kay Barnham
London, UK: Arcturus Publishing
2016
Pg. ?:
Did you hear about the actor who fell through the floor? It was just a stage he was going through.
 
Facebook
Tycho Talent
May 24, 2016 ·
Did you hear about the actor who fell through the floor?
He was just going through a stage.
 
3 September 2016, Fraser Coast Chronicle (Maryborough, Queensland), pg. 14:
DID you hear about the actor who fell through the floorboards?
He was just going through a stage.
   
Reddit—Jokes
Did you hear about the actor who fell through the floor?
submitted February 11, 2017 by MySistersDad
It was just a stage he was going through.
 
Reddit—Cleanjokes
Did you hear about the actor who fell through the floorboards? (self.cleanjokes)
submitted February 5, 2018 by madazzahatter
He was just going through a stage.