People who utilize one device/machine, designed primarily for something specific, when they could just as easily use something else.
Case in point: people using one side of a cable cross machine for abs or triceps when there are plenty of stations available for those exercises, thusly forcing me, who wishes to do cable flyes (or something else involving both limbs), into either waiting until they finish their 12th set or requesting to work in and slow down my HIIT process.
And this is at Gold's Gym Monterey Penninsula! Grrrrr....