I should be the last person to complain about proofreading. Even though I do proofread, I miss typos at an alarming rate. Then again, you don't know how many times I've inserted a typo just to get a comment.
However, when your sign specifically brags about attention to detail, you'd better look it over. This one contains both a typo and a grammar error. So was it a drunk sign painter or a very good Photoshop? It's my favorite picture out of Buzzfeed's list of 27 Reasons Why You Should Always Proofread. I also like Mom's response to a knock knock joke.