i remember an instructor mate of mine telling me this cracker.
the FI told a student ( who was none to bright) to preflight the helicopter thoroughly and check all compartments
he went out to do a lesson on low level quick stops and down wind approaches.
The lesson went ok but my mate sat in the pilot room after getting back and wondering why the helicopter was very very sluggish and needed max power to recover from doing all the manouvers when both he and student were light and carring a light fuel load.
the next student walked in, went out and did a preflight on the same chopper, walked back in with both the FI and dopey student sitting there and asked if he should take out the 70 kg of dumbell weights under the pilot seat used for on a solo 2 lessons earlier.
cheers BP