You may not talk like that . . .
Several weeks ago I passed this restroom door, which was located in a grocery store.
Cannot? Oh yeah? That sign made me want to prove the sign wrong by carrying tons of merchandise into the restroom until it was completely full of merchandise.
I don't know why the "can" vs. "may" error gripes me so much--perhaps it's because I hear and see this problem so often, and also because it seems that it would be so very easy to understand the problem and stop making the error.