Responding from the memory of the thrown switch (or even pure imagination) rather than the actual checking of its current position. I agree it's a problem. But doing it by 2-pilot challenge and response is no guarantee of fixing it. The responder can still respond from imagination, but at least now both pilots (as opposed to just one) would have to slack off to let a problem slip through... so at least it's some improvement.
The worst I remember is one captain, who would respond to the entire shutdown checklist while turned mostly backwards packing his bag. Literally every switch on the list could have been in the wrong position and it would have stayed that way.