Bar Entry Mishap

This is a classic pun, playing on the phrase 'walk into a bar'. The literal meaning suggests the men physically walked into the structure of a bar. However, the punchline plays on the idiom, 'walk into a bar', meaning to bump into something, and the implication is that the third man should have been wary to avoid a collision. The humor comes from the unexpected twist of using the phrase in a physical way.