When you and someone else are walking towards each other in an attempt to pass each other. You move to one side to pass them; they move to the same side to pass you. You move to the other side so you can escape the situation; they try to do the same thing. You are roadblocks of each other until one of you decides to stop and let the other walk by. Regardless of any ill will by either participant, the incident can be really, really annoying.
Me and Stan's mutual roadblock lasted for twelve seconds. Then we each thought the other was being an asshole. We worked it out, though.