I agree with Jon and others that crib sheets are a bane.
The biggest problem as I see it is that if the students are allowed to make
a crib sheet, they spend all their time getting *everything* on the sheet
and then think they *understand* the material. And as Jon says spent their
test time trying to fit an equation on the sheet to the question asked --
never once trying to sort out the physics. Their brains seem to think (that
may be a bit optimistic) "Where do I stuff the given numbers?"
I have found it valuable in this regard, to ask several questions about one
scenario and give more data than is needed such that they MUST think through
the physics.