A trombonist joke has been cited in print since at least 1992:
 
“There is a frog driving east and a trombonist walking west. What can be surmised from this? The frog’s probably on its way to a gig.”
 
The frog and the trombonist joke has been told in several different variations.
 
       
Google Groups: bit.listserv.allmusic
Humourous Musical Jokes
S. Welton BCC
3/30/92
(...)
I’m a trombonist myself, but there was a frog driving east and a trombonist walking west.  What can be surmised from this?
The frog’s probably on its way to a gig.
 
Google Groups: rec.humor.d
Musical Jokes
Vinay K. Chaudhri
4/30/92
(...)
There is a frog driving east and a trombonist walking west.
What can be surmised from this?
The frog’s probably on its way to a gig.
 
Google Groups: rec.music.makers
Trombone Mailing List? Does any exist?
Stuart Hosking
9/17/94
What is the difference between a frog on a bus and a Trombone player on a bus
The frog is More likely to be on his way to a GIG!!!!!!!!!
   
Google Groups: alt.callahans
Liking Drummers
Eric Schwartz
8/28/97
(...)
Q: What’s the difference between a frog driving a car and a trombonist driving a car?
A: The frog’s probably going to a gig.
 
Google Groups: alt.music.trombone
Bad Trombone Jokes
Al
9/21/01
(...)
Q. What’s the difference between a dead frog on the road and a dead trombone player on the road?
A. The frog may have been on the way to a gig.
 
Twitter
Rocking Jamboree
@RockingJamboree
@mikelombardo #musicjokes What’s the difference between a frog and a trombone player driving a car? The frog might be on his way to a gig!
5:02 PM · May 22, 2009·Twitter Web Client
 
Twitter
Hanan Rubinstein
@hananmusic
What’s the difference between a frog and a trombonist crossing the road? The frog might be going to a gig…j/k trombone players… haha
6:09 AM · May 14, 2010·Twitter Web Client
   
Twitter
Clayton Bolger
@claybo76
What’s the difference between a frog and a trombone player waiting at a bus stop? The frog is probably on his way to a gig. #musojokes
11:29 PM · May 10, 2011·Twitter Web Client
   
Twitter
temple8
@temple8
Q: A frog and a trombone player are both driving down the road. What’s the difference?
A: The frog is probably on his way to a gig!
9:53 PM · Apr 10, 2014·Facebook
   
Twitter
Anna
@anna_stro
What’s the difference between a dead frog in the middle of the road and a trombone player? The frog was on its way to a gig. LMAO
1:20 PM · Apr 12, 2014 from Woodland Park, NJ·Twitter for iPhone
 
Reddit—Jokes
Posted by u/JaWiCa July 5, 2020
What’s the difference between a trombone player and a dead frog on the side of the road?
The frog was probably headed to a gig.