I dunno. This is one of my most frequent errors. I form complete sentences in my head, and my fingers trascibe them aurally as I type. I type in whole-word units, and sometimes it just doesn’t come out right. Our/are and you/your are particular betes noires.
But in the particular situation these people were in, I would have brought in an educated ringer to proof the sign.
I should also say that my keyboard is full of cookie crumbs, and some of the keys have to be pressed really hard...