A "wet" takeoff of a KC-135 with J57 engines—By USAF Photographer (USAF photo) [Public domain], via Wikimedia Commons
Why is water-contaminated fuel bad, but water-injection is not?
The way I understand it, water-injection lowers the combustion temperature, which is good and bad, good for cool yet increased power-output / mass flow, bad for wasting [unburnt] fuel.
Why is then water dangerous when a trace amount of it is found in fuel? What's the worst that can happen apart from black exhaust and higher power?
Water-injection in jet engines and piston engines is mixed with fuel before it enters the combustion chamber, so I'm thinking fuel already mixed with water should not be any different. Of course I'm wrong. But why?