I cut through the MIT Medical building today on my way to class, and came upon a revolving door next to a normal one. A nearby sign read “To conserve energy, please use revolving door.”
In black magic marker, someone had scrawled under it: “Energy is always conserved.”
Ahh those witty Physics majors…
Heh. At CMU, every year they fertilize the grass on the cut. The grounds keepers always block off the area with cones and yellow caution tape. And every year, someone comes allong and politely adds a sign saying “Please Keep Off The Shit.”
This is generally next to a sign that says “Do Not Lick.”
Very valuable advice, there… 
Not to mention the latest in MIT Dome hacks. Very nice, but it’s not as cute as the R2D2 one.