I got the tl;dr out of the way in the title.
I've written previously about the value of multiple points of view (literal points of view in this case, but I think it's valuable for figurative points of view, too). Last night, in Game 4 between the Boston Red Sox and the Houston Astros, was another example.
Here's the situation as it was in Houston (the location is kind of interesting, though not really important to the ruling). It's the bottom of the first, and the Astros are already down 2–0, but they have George Springer on first after a one-out single, and Jose Altuve up to bat. Altuve hits a deep fly to right, and Red Sox right fielder Mookie Betts reaches up and seems about to make the play, when his glove is closed shut by a fan's hand. The ball bounces back into right field, where Betts retrieves it and fires it back into the infield. Altuve ends up on second, and Springer (who presumably had to wait to see if Betts made the catch) stands on third.
Umpire Joe West initially calls a home run, and then appears to indicate interference (as shown here at the 8:48 mark). The umpires collectively go to the replay, and after a delay of a few minutes, they call Altuve out, and order Springer to return to first. After Marwin González is hit by a pitch, Yuli Gurriel flies out more conventionally to right and the Red Sox escape without further damage.
In the aftermath of the Red Sox' 8–6 victory, however, there was considerable controversy over whether the interference call was the right one. The ruling was that because Betts's glove did not exit the field of play—that is, it did not cross the imaginary plane of the outfield fence—he was interfered with. Had the glove been beyond the fence, then any contact with the fans would not have been considered interference.
The problem is that it's far from obvious where Betts's glove was at the moment of contact. The Red Sox observed (as did some others) that Betts's body had yet to reach the fence, but the Astros pointed out that Betts was reaching backward for the ball. Both sides agreed that the ball would have gone into the stands were it not for Betts, and both sides agreed that Betts had a good chance of catching the ball. (I've seen a few fans claiming that Betts simply closed his glove early, but neither I nor any professional commentator seems to find that credible. See here at the 0:45 mark for a pretty clear video of Betts's glove being closed by a fan's hand.)
Incidentally, whether Betts would have caught the ball doesn't have any bearing on the correct call. West's call was predicated only on whether the fans interfered with Betts's fielding in the field of play. The approved ruling associated with Rule 6.01(e) reads:
If spectator interference clearly prevents a fielder from catching a fly ball, the umpire shall call the batter out.
The comment on that rule goes on to clarify:
No interference shall be allowed when a fielder reaches over a fence, railing, rope or into a stand to catch a ball. He does so at his own risk. However, should a spectator reach out on the playing field side of such fence, railing or rope, and plainly prevent the fielder from catching the ball, then the batsman should be called out for the spectator’s interference.
That's what made the correct interpretation of the replays so vital.
Nevertheless, both sides also thought the replays confirmed their conclusion, each perhaps pretending to a greater certainty than they really felt. They're really not that conclusive either way, at first glance, and it was important, probably, that the call on the field was interference. Here's a shot from one angle, for instance (the left-field camera, I think):
Can you tell where Betts's glove is in relation to the fence? I can't.
Well, we don't have to tell from that shot alone. Here's a second shot from another angle (maybe the first-base camera):
Hmm, it's not obvious from that shot either.
Once again, though, we don't have to rely on either shot in isolation; fortunately, the two images together will tell us what we need to know. Both shots show the play a split-second after the fan had made contact with the glove, and with the ball just about to strike the outside of the glove. The fans are still looking up because they're not trained to follow the ball into the glove, and because that baseball is moving fast, but that white blur is the ball in both photos.
How does this help us? Well, let's take a look at where the glove is in relation to the wall. Here are the same two shots, but with the same location marked on the outfield wall padding:
Notice where the glove is in relation to that mark in the two images. It's to the right of that mark from the point of view of the left-field camera, but it's just about in line with the mark (or maybe a little to the left) from the point of view of the first-base camera. It's simple triangulation: If the glove is directly above the fence, then it should be in the same position with respect to the mark from both views. If it's in front of the fence, it should appear further to the right in the first view (from left field), and if it's beyond the fence, it should appear further to the left in the first view.
Since it's further to the right in the first view, the glove must have been in front of the fence at that moment, and the interference call is the right one. (I was mildly surprised to discover this, by the way. If I had to guess, I would have guessed that the glove was beyond the fence—but I would have been pretty loathe to guess.) Without knowing more about the location of the cameras relative to the wall, we can't be sure how much in front it was, but at any rate, the contact was made in the field of play.
ETA: Here's a third, intermediate view—from the third base camera, I think—further confirming the findings:
