Today at lunchtime I saw the following sign on a building – it looked, at first glance, like the very official “No Parking” sign above it. Untuil you read the message
[spoiler] **Do Not Block Loading Zone
The Parking Monkeys Have No Mercy**[/spoiler]
I saw a parking lot at a church in Blacksburg, VA (convenient to both downtown as well as Virginia Tech) that had something to the effect of:
Thou shalt NOT PARK!
Church parking only.
There’s an add painted on a brick building in Brooklyn for a carpet store:
Say No to Drugs
Say Yes to Rugs!
I used to live next to a building in San Francisco that had the ghostly remains of a professionally-done painted sign on it. Visible only from the 8-foot wide fire escape area between buildings were the enigmatic words:
Roller Canaries
and nothing else.
Apparently long, long ago, that building held a pet shop. At the time, I had visions of little birds on skates. Now, with the internet, it’s easy to discover that roller canaries are just a variety of small birds.
When I was younger I backpacked across SE Asia and among the places I visited was this site:
On the wall was a sign (in several languages) that read:
“Falling into a Trance in the Snake Temple is Prohibited”.
That has got to be the oddest totally serious “official” sign I’ve ever seen. 
Chili’s usually has signs on their short-term parking for takeaway orders that go something like this:
“Chili’s-To-Go Parking Only – All Others Will Be Crushed And Melted”
Ha! There is the same sign at a church in Winchester, VA. Probably spiritually related 
I remember that sign from my time at Tech. It’s at Blacksburg Baptist, if I remember correctly. I love that one.
I’ve seen a couple in various stores with messages urging parents not to leave their children unattended. My favorites are “Unattended children will be sold to the circus.” and “Unattended children will be given a shot of espresso and a free puppy.”
The roller part describes their type of song. A long, rolling trill. It must have been a specialty of the shop to merit it’s own sign. So instead of roller-skating, they probably held lorgnettes and looked down their beaks! 
I’m reminded of the time my brother, sister and I were at a pet shop and saw a sign about “Robo Dwarf Hamsters”. The internet reveals that this is a relatively common breed, but we still had a good time picturing very small (and possibly bearded) mechanical hamsters.
My favorite.
Many years ago, my cousin and I were out riding our horses on a counrty back-road. There was a nice brick house with a built on garage, and spray painted across the garage door, it said: Nothing for sale- Don’t ask.
There was a man in the front yard doing yard work, and no matter how big the temptation, we didn’t ask.
You should never park here.
That is indeed very funny.
Not terribly long ago (within the last 20 years), I saw a series of Burma Shave signs. They said:
Thirty days hath September
April, June
And the speed offender
Burma Shave
For unofficial (i.e., graffiti) examples, these are my favorites:
“Wobbly rockets”
“Where is my paranoid kitty?”
“Is the glass half empty or half full?”
to which someone else wrote:
“I think the glass is twice as big as it needs to be”
Mine is graffiti added to a stop sign:
Don’t
STOP
Believin’
I saw a perfectly professional looking sign at a parking lot in Des Moines, Iowa, with the following text:
I think that this one, from a welsh railway station, is probably a fake.
But it just might be real.
British Rail. Always promising, never delivering.
I saw this sign in a yarn shop and asked the woman how long I needed to leave my kid unattended before they gave him his puppy. I thought she would laugh, but she was not amused.
