Since there are 120 hi-lo plus cards in a six-deck shoe, and 72 neutral cards, it is theoretically possible to have a running count of +120 after three decks were dealt. If you run into one of those, stop the game so you can get a second mortgage on the farm, and then bet it. :-)
If you got such a count, there are 3 possibilities:
1. All of the remaining cards are 10s and Aces.
2. You have made a counting error.
3. The House is cheating.
I would suspect 2 before 3, but both are more likely than 1.
If you had such a shoe, you are not quite certain to win. The House could draw a back-door blackjack, against your 20. (If they have an Ace up BJ, you would insure and break even on the hand.)
Here is an exercise for the mathematically minded: What is the probability of getting an RC of 120 in an honest 6=deck shoe? I am tired tonight, but I can post the answer tomorrow if anybody is interested.