Good gym etiquette demands that you put the weights/plates back after you use a machine or a barbell or whatever when you're through... unless someone's waiting for it and you got a fairly good idea they're going to need some of that weight left on there and you ask them, makes sense right ?.. but what if you go to jump on the machine and someone left a pair of 45s on... after you add the additional plates that you need to finish the exercise when it's time to clean up do you put the 45s that were there in the first place back also, the ones left there by sum1 NOT cleaning up after themselves ???