As a Tower controller in UK about 20 years ago I had a B752 depart with a massive fuel leak, after about a dozen attempts to get the crew to realise they had a problem, "No indication of a problem in the cockpit" my self and the radar controller kept getting told!!
After they realised they had a problem they returned and had to use reverse on landing due to high weight, the whole of the aircraft was engulfed in fuel vapour!! I remember a helicopter pilot sitting rotors just accidentally broadcast
It was a common 'ish' fault with fuel line on RB211-535 at the time!