I took this photo on April 24 of this year:
It's a graffiti production by the AIDS (= Alone in Deep Space | America is Dying Slowly) crew in Jersey City. As you can see it's been treated harshly by the weather, with the right half badly washed out. But no one's gone over it, which typically happens with old graffiti. Yet other walls in that area – there are over a dozen in close proximity – have been gone over many times. What's the difference?
Perhaps it is that this wall is just a bit more exposed than some of the others, and so you're a bit more likely to get caught. But I think mostly it's a matter of respect for the writers and the crew.
Here's what that wall looked like the first time I flicked it, on Nov. 28, 2006: