Rule number one, if you're going to send something you wrote out into a public forum: PROOFREAD!
You don't have to adhere to every last obscure rule of punctuation. But if you're going to, say, a rally to protest your government and you've taken the trouble to paint your back window or buy a piece of posterboard and write on it a witty putdown you concocted yourself and maybe decorate it with a few squiggles of red, white and blue, take some pride in your work and check the spelling!
Better yet, sketch out your words in pencil first and then check them before you do permanent damage to your posterboard.
Geez. Sometimes Tea Partygoers make it too damned easy.
For more, follow the jump or check out Teabonics, the brilliant Flickr collection from whence all these photos come.
Remember, proofreading marks are your friend!