I do it all the time...but then I'm on the other side of the checkout, so I can get away with it.
We sell mood rings and I love telling them that the rings are special and if you squeeze them hard enough they disappear and then find them in strange places (pen lid is my favourite just now...).
Almost feel bad when they walk out with a red face from trying to squeeze it so tight. Almost...
Don't even get me started on how much fun I have with a squeaker. I told a group of kids that if they could find the squeaky one of
these they could have it for free. They spent ages raking through the box. Again, almost felt bad...