If you are using an alkaline or neutral fix and no stop. And if you use static water for the rinse after the developer and not running water, you can fog paper if you turn the lights on too soon. You see, developer gradually builds up in the static rinse and then is carried into the fix which then becomes a developer with a silver halide solvent in it, which assists in fogging the paper and developing it further. Light only adds to the injury.
So, if you use an alkaline fix, make sure you use a stop or running water for a rinse.