Holding down 1-5-9 or 3-5-7 can drop a POS card reader into debug mode, but it's brand dependent. (I have had it also crash the host terminals in some cases as well.)
also want to say that this is illegal in most places. The store may or may not press charges, but they have the right to and they will win that case if they do. So only do it if you know you can get away with it or have permission or don't mind having the stain on your legal record and whatever fine they hit you with
It's not destructive, nor can it really be considered illegal access. I suppose there is something illegal about it if it's classified as a DoS, I guess. There isn't much on a payment PED aside from an encryption key and maybe some network information.
Entering your PIN and accidentally hitting 3 buttons hardly seems like something to get charged with. How you leverage that for other things could easily be a problem.
All I am saying is that the likelihood of serious repercussion is very low. Still, I have to agree: Only do it with permission and with purpose.
It serves a niché function in some situations. (Physical pen testing, in my case. I needed the person at the register gone in one case so I could setup a small network tap.)