This flaw, known as OHSS by the creatures community, is the One Hour Stupidity Syndrome, which would inevitably effect the majority of the norn population within a game. Basically, with no other way to say it, a norn afflicted with the syndrome would be stupid. It would no longer listen to commands, stop attempting to learn words, would eat at odd intervals, attempt to breed with any norn that came within reach, and would be unable to navigate beyond it's current location, which would usually be the nursery it was born in.
So, after a few of these OHSS norns were born, they would just keep breeding with each other (since they all remained in the same nursery) and producing more OHSS norns, until there was nothing but stupid norns that couldn't be lead to explore the world, effectively ending the game.
OHSS was found to be caused by a defective genome, which increased the size of the concept lobe and strengthened the dendrites between the concept lobe and the decision lobe. This made it so that chemicals responsible for pleasure and pain increased in production and did not decay fast enough, building up in the brain.
A Norn with OHSS, if rewarded for performing an action or learning a word, would not have the pleasure chemical decay fast enough, so that any subsequent actions the norn would take (no matter what they were) would be equally rewarded. After being mentally rewarded by more pleasure chemicals, the next action would likewise seem to be rewarded, ad infinitum until the norn's brain literally turned to mush within an hour.