User blog comment:Neowitch/680 - "Two men walk into a lab..." Stop me if you know that one!/@comment-4754162-20120906042906/@comment-4493594-20120906102148

Actually, it goes "Two guys walk into a bar, and the third one ducks."

It's a play on words. "Bar" does not refer to the kind of bar where you can sit down and order a drink but a "bar" as you use for limbo or in high jump. This guy explained it rather well, despite poor drawing skills: http://seqp.blogspot.de/2008/12/two-guys-walk-into-bar-and-third-one.html

Now, a lot of people don't get the joke and don't remember what it's about so there are a lot of variations of this joke in which the wordplay is dropped and some sort of alcohol joke is made instead.