I think it was the one in the bottom of the fifth, Twins down 5-0. Runners on 1st and 2nd, and Polanco checked his swing on 1-2. Astros claimed it hit his bat and was caught, which would have retired him. Instead, it was called a ball and Polanco worked the count for a walk, loading the bases.
It's a call which isn't reviewable (which has its own questions -- why isn't it reviewable?), so Houston couldn't challenge. It was almost imperceptible, but there appeared to be a slight change in direction, and I think (and Hosken and Woof seem to agree) it would have been reversed if Houston could have challenged.
Alas, Kepler and Lewis each struck out, but had either of them gone deep and the Twins come back to win, Astros Daily would have had multiple posts about the topic.