It depends on what the contaminant is.
If it is water, see Zaid's answer.
If it is flammable liquid the damage depends on how it behaves in the fuel pump, injectors and combustion chamber. It may cause no effect to jamming the injector(s).
If it is solid contaminant, it will be caught by fuel filter or may stuck in the injector.
In all cases (except for large particles) you may get cylinders to fire earlier, later or miss a fire.
On the other hand, one tank of contaminated fuel won't kill the engine; you need to use that fuel for longer times to get into serious trouble. Or it must be really, really bad fuel - contaminant with small amount of fuel. The older engine you have, the worse fuel it can digest without harm.
Watch for misfires and unexpected power output (significantly lower, significantly higher) and throttle reaction.
Run the tank to the limit, fuel 5 litres of trusted petrol and run it to the limit again, then fill it up with trusted petrol.