Geisel Library, 8th (top, smallest, great-window-view-with-sunlight-streaming-in, most beautiful) floor, men’s bathroom:
One student kept a running tally of “Number of Times I’ve Had Sex on the 8th Floor”. It got to about fourteen or so and then an arrow connected the last one to the note, “Got caught. Didn’t stop until I was done.”
Another student wrote under the whole mess, “It doesn’t count if you were by yourself.”
How about “What are you looking up here for? The joke is in your hand!”
or
“Hope you enjoy these attempts at humor. By the way, you are pissing on your shoes.”
My favs are:
Free photos of the president, wipe to develop
and
This is where the dudes hang out
Often seen in truck stop stalls in the early 80’s:
Jimmy Hoffa, call your dispatcher!
In very, very tiny handwriting on the bottom of a toilet stall door:
You are now shitting at forty-five degrees
Not from in a public toilet (though it may as well have been), but from a railway station, comes one of my favourites:
KYLIE IS A MOLE AND A HOAR
I saw one that said:
I LOV[del]B[/del]E BUSH
Written right at the bottom of the stall door:
BEWARE OF LIMBO DANCERS!
[QUOTE=UncleRojelio]
Here I sit broken-hearted,
Paid my dime and only farted.
When I was a kid, there was an issue of Mad Magazine that had a bunch of classic bathroom graffittis on a sheet of stickers all ready to paste to your favorite bathroom wall. These continue to be the best lines I have ever read.
[/QUOTE]
Mad also ran a series where they “cleaned up” graffiti - yours turned into:
Here I sit happy hearted,
Talks on Mid-east peace have started.
This was an edition from the early 80’s 
Grim
In the restroom of the main offices where I used to work.
Jesus is coming, are you saved?
And underneath,
If not Jodie in HR can put you on direct deposit.
IMNSHO, the classic is:
He who writes on outhouse walls
Rolls his shit in little balls.
Those who read these words of wit
Then eat these little balls of shit.
The funniest response I’ve ever seen to all those “Here I sit…” jokes:
Here I sit
Happy and heartful
Came to shit
And laid a cartful!
In large, bold script:
I Kill Children!
Underneath, in a different hand:
Great! I eat children, maybe we can work something out!
[QUOTE=Gorsnak]
Written right at the bottom of the stall door:
BEWARE OF LIMBO DANCERS!
[/QUOTE]
This made me giggle for several minutes.
On the sanitary napkin bin inside the stall at Stephens College in Missouri: Surfboards and harpoons go here.
Underneath in a different hand: Nice euphemisms!
“Bill O’Reilly eats little boy jizz”
Please do not throw toothpicks in the toilet,
the crabs have learned to pole-vault.