Naomi has
Chalice of the Void in play with 3 charge counters on it.
Abra plays a 3 CMC creature illegally (both players don't remember about the Chalice).
Naomi takes her next turn. Abra takes his turn and copies the 3 CMC creature with a 4CMC copy spell.
Naomi takes her turn… then passes her turn back to Abra but says “woah woah wait.. you should never have been able to cast that 3CMC creature! It should have been countered!”
In these 4 or so total turns that have taken place there has also been changes to life totals etc so it's quite difficult to roll back. JAR says to fix anything currently illegal… While the 3CMC creature is not “currently illegal” it was illegal when cast.
What is the proper way to resolve this situation?
Thanks!