Often I’ll drive by one of those signs with the movable letters and see that a new message has been created, with the spacing correctly laid out. Then after a few days or weeks, the spacing goes all squirrely. For example, it’ll start out like
PORK CHOPS $2.99/LB
and by the end of the week it looks something like
WE EKLYS PECIA L
PORKCHOPS$ 2.99 /LB
How do the letters wander around like that? Even if they only sat loosely in their tracks, it doesn’t seem like the wind, or traffic vibrations, would be enough to jostle them around, given that they are pretty stable for at least a few days. And I can’t imagine it is done intentionally; a joker wouldn’t horse around with the spacing, he’d make anagrams like SWILL $2.99 or something.
The answer to this question is probably obvious to someone who has set up a sign like this. I haven’t, and I don’t know how the letters are mounted or moved, so it is a complete mystery to me.