Wigan Wire game in 70's when two balls ended up on pitch and the Pie Eaters picked up the wrong ball when both balls were bouncing around in open play and scored and the ref allowed it.
Or did I dream it ?

Yes, and got away with it too! Although thought it was for Hull and against Leeds

Shane Cooper did something similar for Saints v Widnes many moons Bach. He picked up a 2nd ball when defending and dropped it when going into a tackle - Saints got the head & feed!

this happened a couple of years ago at a haven v leigh game, duffy came on for leigh and for 1 minute they had 14 players on and scored a try, ref realised and sin binned duffy, haven got beat and complained to the RFL who investigated and said no case to answer even though duffy had entered the field of play illegally and not gone through the 4th offical