There's a huge advertisement in Minneapolis just off the U of MN campus for the "lovepower church," that I have always thought an eyesore- and it can be seen from 35W.
Just saw that the advertisement, which is supposed to say "LOVE POWER," has been vandalized in an amusing fashion with a single letter.
My favorite bit of vandalism, though, was in Syracuse.
The billboard originally said: "Grafitti is a Crime!" Some clever tagger spray-painted the word "FUN" between "a" and "crime," so it read for months as: "Grafitti is a FUN Crime!"