The only time I've had fuel go off in a big way was an MGB under an 18 month refresh. I'd only left a couple of gallons in, deliberately, because I figured that was better than having a full tank go off.
Those two gallons went off spectacularly. It didn't even smell like petrol and because it had evaporated in the lines, left a sticky green deposit on everything so it took some shifting. Then I had to drain the tank. Because the B tank has the sender on the side, it's possible to remove the sender and shine a torch to inspect the interior. What that showed was a patch of rust, directly under the fuel pipe, because below the take off pipe is a low point. Also, because the front edge of the tank is also a low point, there was a line of rust. The rust is either water in the petrol, or condensation.
With an injection engine it would be a lot more difficult to sort out bad fuel.
