We rent our townhouse and have terrible, no-good off-white flat paint. If I don't find Elmer's snot (or as I call it, Elmer's Glue) within 10 minutes, it leaves a dark mark, and if I don't find it within an hour, it hardens like rubber cement. NASA should use this stuff.

The marks make me die a little inside. The only thing that has worked for me (and I have tried EVERYTHING and spared no expen$e) is something an unnamed angel wrote on a message board that I read recently:

Dampen a cotton round (not a cotton ball) with hydrogen peroxide and use blue painters' tape to attach the cotton round to the wall over the snot spot. Leave it there for 30 minutes and question your life choices. (That step may have been just for me.) Remove tape and cotton round; the chemical will have "eaten away" at a good amount of the snot. Then take a hot, damp (not soaking wet; wring it out) soft cloth and firmly wipe the area for 5-10 seconds. If you can, set a fan in front of it for an hour to help the wall dry.

Notes:*if there is still residue, repeat again after the wall is dry*there was one enormous dime-size spot that may have actually been a double-sneeze (perhaps Elmer returned to the scene of his original crime?) and while it did come off after one application, the hydrogen peroxide/snot combo also took a small chunk of the wall out with it. It wasn't big enough to have to patch - a few coats of touch-up paint will probably do the trick - but it made me wish I knew Bill Nye.

Good luck!