Standing on the corner, all alone with that look of desperation in their eyes. If you make eye contact they see the opportunity to start talking and it can be awkward to just keep walking. "No, thanks. I'm not interested. Really, I'm on my way to a meeting." If you look over your shoulder as you walk away you can see that disappointment and it's easy to think, "Man, they are just so young." I hate poster sessions.
I know that they are an important part of a conference to allow people who were not selected to give a talk the opportunity to share their work, but is really worth the amount of time spent making the poster to stand in front of it for a couple of hours in a busy room and have a couple of people actually read it and interact with you? I say no. I have had the good fortune to have been given the opportunity to talk at all of the meetings I have attended (many of the early ones were small meetings) and have only made a single poster, in a year in which I wanted to present on two separate topics at the same meeting. From that point I vowed to avoid the poster in the future. I would rather give a talk with six people in the audience - which I have done at a giant meeting in which my session was a bit outside the main theme - than stand alone in a crowded room.
On the flip side, I don't enjoy going to poster sessions either. If it's a small meeting and under 100 posters it can be done, but larger meetings with hundreds of posters just make it a game of finding a needle in a haystack. Do I want to browse 400 abstracts to find a couple I might be interested in? Hmmmm, no. The alternative is going to each section (if the meeting is well organized) you might be interested in and browsing through the crowded maze of audience members and poster presenters who are over-eager to spill their spiel all over you. Maybe there are one or two posters that are exciting and you have the opportunity to talk with the students and see what direction their work is going, but more likely you end up hearing a dozen stories that you do not want to, simply because you get trapped near a poster and your eyes linger too long on the title. I still go to the poster sessions in the hopes that I'll find it to be worthwhile one day, but invariable end up walking out covered in regurgitated information all over my good conference jeans.