People who don't know how to walk. The sidewalk isn't that big, and people need to get to class. Either (a) walk at a reasonable speed, or (b) get the hell out of the way. It's brutal in the spring/fall when the grass is wet near the science buildings, and people decide to walk as slow as humanly possible down the concrete paths in a zig-zag pattern while texting so you can't get around them.