Here is the sentence again:
Jenny noticed a greasy snail-like trail that snaked from the kitchen counter
onto the floor and then over to the hallway, June,
Jenny's cat, had stolen another stick of butter and tried to drag it to
the laundry room.
Jenny noticed a greasy snail-like trail is a main clause. June, Jenny's cat, had stolen another stick of butter is the second main clause. Two main clauses cannot have a comma alone connecting them.
If you change the comma after hallway to a semicolon, you will correct the problem.