Have had several. Some I do remember for the moment:
- B727 fuel tank leak. Was visible by dripping at a drain mast. Inside the tank is a big black sack, called a bladder fuel tank. One of the connections between bladder and pipes were leaking. I had the opportunity (was the only person slim enough to fit into) to go into the manholes and thus inside the bladder. With special tooling of course and a mask, no metal tooling or equipment working on electricity. Also opened 2 manholes, one with a fan on it sucking fumes out, the other preventing to suck the tank vacuum.
Leaking was permitted to do 1 flight to the homebase I believe. (very cool to see grafitti on the inner tank walls "x was here"!!)
- B757 huge spill through the fuel tank vent (naca inlet) on the wingtips. Occured due to a malfunctioning shutoff valve. So during fueling the tanker just pumped in untill it came out of the vent. Huge spill, so firebrigade was called to clean.
- A300 engine fuel leak (out of drain masts) during engine working only. Was a loose supply pipe on the engine.
- Typical problem on several types: most modern pax aircraft engines have a small drain cannister on the engine, which collects fuel that is still in the supply pipe after shutdown. This collected fuel is normally used first during the next engine start. Sometimes the valve on this cannister is stuck or broken and when the cannister is full the fuel just goes through the drain mast. You'll see a puddle of fuel under the engine after shutdown, or see it leaking during engine crancking (even without fuel on). But not that serious to call firemen.