AP
After losing a challenge earlier, the Giants and Diamondbacks were tied in the fourth inning when Arizona's AJ Pollock scored on a passed ball.
Replays clearly showed that Pollock was out.
However, since the Giants lost their previous challenge, they were unable to ask for another review. In addition, umpires are not permitted to initiate reviews until the seventh inning.
The Giants went on to lose 5-4 with that missed call being the difference.
An easy solution to the problem is to keep the challenge system the same, but to give the umpires the power to initiate reviews throughout the game if they feel it is warranted.
You can see both plays in this video. The play at the plate comes at the 1:11 mark.