“A guy walks into a bar…”  is a typical form of what has been called the “bar joke.” A version involving Russian physiologist Ivan Pavlov (1849=1936) is:
 
“Pavlov walks into a bar. His phone rings, and he says, ‘Damn, I forgot to feed the dog.’”
 
“Pavlov walks into a bar. The barman rings the bell for last orders. Pavlov immediately demands food” was posted on Twitter on August 30, 2010. “Pavlov walks into a bar. The phone rings and he shouts ‘Shit, I forgot to feed the dog!’” was posted on Twitter on June 25, 2013.
 
   
Wikipedia: Ivan Pavlov
Ivan Petrovich Pavlov (Russian: Ива́н Петро́вич Па́влов; IPA: [ɪˈvan pʲɪˈtrovʲɪtɕ ˈpavləf] ( listen); 26 September [O.S. 14 September] 1849 – 27 February 1936) was a Russian physiologist known primarily for his work in classical conditioning. From his childhood days Pavlov demonstrated intellectual curiosity along with an unusual energy which he referred to as “the instinct for research”. Inspired by the progressive ideas which D. I. Pisarev, the most eminent of the Russian literary critics of the 1860s, and I. M. Sechenov, the father of Russian physiology, were spreading, Pavlov abandoned his religious career and devoted his life to science. In 1870 he enrolled in the physics and mathematics department at the University of Saint Petersburg in order to study natural science. Pavlov won the Nobel Prize for Physiology or Medicine in 1904, becoming the first Russian Nobel laureate. A Review of General Psychology survey, published in 2002, ranked Pavlov as the 24th most cited psychologist of the 20th century. Pavlov’s principles of classical conditioning have been found to operate across a variety of experimental and clinical settings, including educational classrooms.
 
Twitter
Dean Burnett Xmas
‏@garwboy
Pavlov walks into a bar. The barman rings the bell for last orders. Pavlov immediately demands food. #sciencejokes.
1:23 PM - 30 Aug 2010

Twitter
Dean Burnett Xmas
‏@garwboy
@endless_psych Pavlov walks into a bar. The barman rings the bell for last order, Pavlov immediately orders food.
5:57 PM - 26 Sep 2010
 
Twitter
Kevin Duggan
‏@KervinDurggan
Pavlov walks into a bar. The phone rings and he shouts “Shit, I forgot to feed the dog!”
5:56 PM - 25 Jun 2013
     
Reddit—Jokes
Pavlov walks into a bar.
submitted July 29, 2013 by crow_man
Ivan Pavlov walks into a bar. The bartender rings the bell for last drinks, and he thinks “shit - I forgot to feed the dog”
 
Reddit—Jokes
Pavlov is sitting at a bar…
submitted May 1, 2014 by OldMajorr
...when another patron walks in and a bell on the door rings. Pavlov stands up, says, “I forgot to feed the dogs,” and leaves.
 
Reddit—Jokes
Pavlov walks into a bar and hears a loud bell.
submitted February 9, 2015 by mikhel
“Oh shit,” he exclaims. “I forgot to feed my dogs.”
     
Reddit—Jokes
Pavlov walks into a bar
submitted March 22, 2015 by [deleted]
As he opens the door the bell rings. “Oh shit!” Yells Pavlov, “I forgot to feed my dogs!”
 
Reddit—Jokes
Pavlov walks into a bar.
submitted December 15, 2016 by ktkps
His phone rings, and he says, “Damn, I forgot to feed the dog.”