“A guy walks into a bar…”  is a typical form of what has been called the “bar joke.” A dangling modifier version is:
 
“A dangling modifier walks into a bar. After finishing a drink, the bartender asks it to leave.”
 
The line was written by Eric K. Auld for McSweeney’s on November 8, 2011.
 
“Crowded with happy patrons, a dangling modifier walks into a bar” is another dangling modifier joke. “A dangling participle walks into a bar. Enjoying a cocktail and chatting with the bartender, the evening passes pleasantly” is a similar type of joke.
   
   
Wikipedia: Dangling modifier
A dangling modifier is an ambiguous grammatical construct, whereby a grammatical modifier could be misinterpreted as being associated with a word other than the one intended or with no particular word at all. For example, a writer may have meant to modify the subject, but word order makes the modifier seem to modify an object instead. Such ambiguities can lead to unintentional humor or difficulty in understanding a sentence in formal contexts.

A typical example of a dangling modifier is illustrated in Turning the corner, a handsome school building appeared. The modifying clause Turning the corner is clearly supposed to describe the behavior of the narrator (or other observer), but grammatically it appears to apply to nothing in particular or to the school building.
 
McSweeney’s
NOVEMBER 8, 2011
LISTS
SEVEN BAR JOKES
INVOLVING GRAMMAR
AND PUNCTUATION
ERIC K. AULD
(...)
2. A dangling modifier walks into a bar. After finishing a drink, the bartender asks it to leave.
 
Twitter
Jacob Andreas
‏@jacobandreas
“A dangling modifier walks into a bar. After finishing a drink, the bartender asks it to leave.” http://www.mcsweeneys.net/articles/seven-bar-jokes-involving-grammar-and-punctuation …
10:34 AM - 8 Nov 2011
 
Twitter
Christopher Sopher
‏@cksopher
“A dangling modifier walks into a bar. After finishing a drink, the bartender asks it to leave.” http://www.mcsweeneys.net/articles/seven-bar-jokes-involving-grammar-and-punctuation#.TrmEufZL4fU.twitter … I love @mcsweeneys
4:12 PM - 8 Nov 2011
 
Google Books
Grammar for Grown-ups:
A straightforward guide to good English
By Katherine Fry & Rowena Kirton
London: Square Peg
2012
Pg. 262:
A dangling modifier walks into a bar. After finishing a drink, the bartender asks it to leave.
 
Google Books
The 25 Rules of Grammar:
The Essential Guide to Good English
By Joseph Piercy
London: Michael O’Mara Books Limited
2014
Pg. ?:
A dangling modifier walks into a bar. After finishing a drink, the bartender asks it to leave.