Saw this online and got a good laugh at it. If you're going to go through all the trouble of making a really big sign, and putting it up where everyone can see it, you might want to have someone proofread it for you.
I've seen professionally made signs with obvious typos on them. When I was in Texas, a local restaurant had signs put on the wall of the building, in the parking area, reading: PARCKING FOR CUSTOMERS ONLY
I remember reading The Outsiders when I was about 12. I remember there was one chapter where the narrator was talking about a restaurant he worked at. There was a no parking sign that said, "Violators will be persecuted" and he laughs that it should say "prosecuted". He wasn't sure if it was intentional or a typo. I remember not getting, I didn't understand the difference in the two words. It wasn't until I got out a dictionary and looked them up, and then asked my mother, that I got it.