Back in the days when we had a bit more freedom to do such things, a mate asked me to renew his instrument rating. He had hired an old Aztec from some el cheapo operator at Essendon.
When we boarded there was a very strong fuel smell in the cabin. I got out and gave the wings a good shake and fuel sloshing around in the belly started dripping out under the aircraft. Presumably the cross feed line had broken.
Needless to say we did not switch anything on to check the fuel gauges!
The owner was quite p!ssed off that we grounded his pride and joy with an entry in the MR.