couple from oxford
there's a toilet in the JR hospital. I used to use it whenever I went to pick up a mate who worked there. It had a sign that was repeatedly altered (different pens after each cleaning etc, so you know someone really felt the need).
The weird thing was, the alterations were purely punctuation and made no sense. The altered version read:

Please "do not" put paper towels down the toilet.


Also in oxford are some other classics (only read half the pages so far, so I'll doubtless be glasscocking. Sorry.)

"However beautiful she is, someone somewhere is sick of her shit." - Cellar bar toilets

"if you don't like hockey ya'll can eat a bowl of fuck" - with the addition in a second pen of "I think you'll find you mean y'all - it's an abbreviation of you all, do you see?" if the (oft graffiti'd and oft repainted) Turf Tavern bogs.

Plus all those bloody 12 monkeys signs.

Edit:
There's also a sandwich shop called "heroes" that is permanently changed to "herpes". I like that whoever does this manages to do it to all the signs on the shop (sides of awning etc) as well as to the board they put on the pavement.
( Mockingbird Practitioner of SCIENCE, Fri 4 May 2007, 9:29, Reply)