BOSTON — The custom that pitchers should treat all popups as the plague is ridiculous. And it cost the Orioles a victory yesterday.

Pitchers wear baseball gloves. They have been playing baseball since they were kids, when everyone learns to catch a popup.

But because of the ridiculous custom, Baltimore’s Jeremy Guthrie lost the chance for a shutout in his fourth career start (25th total big-league appearance) and the Orioles blew a game they had in their back pocket.

Give credit to the Sox for rallying. But their momentum was started by a popup that maybe was 25-30 feet in the air and about 25-30 feet in front of the plate, an easy out.

With one out in the ninth and Baltimore on top, 5-0, Coco Crisp hit that weak popup in front of the plate on Guthrie’s 91st pitch.

Guthrie came off the mound and could have caught it in his bare hands, the ball was hit so softly. He took a step toward the ball, but, bowing to custom, he backed off.

Third baseman Chris Gomez had too long a run to comfortably make the play, so catcher Ramon Hernandez tried to grab it. The ball wasn’t in the air long enough for him to toss away his mask and face home plate to account for the spin that takes such a popup back toward the infield.

So Hernandez dropped the mask at his feet, fairly tripping over it, as he tried to reach for the ball with his back to the plate. The spin of the ball helped it squirt out of his glove.

The ball landed on the grass a few feet from Guthrie.

That’s when Baltimore manager Sam Perlozzo’s strategic wheels began turning. He called on his two best relievers, both well rested, and the decision blew up on him and the Orioles in the rubber game of the series.

And it all went back to a seemingly harmless popup.

“We don’t really practice catching those. We leave it to (the infielders). We’d do it the same way the next time,” said Guthrie, remarkably composed given the fact his gem blew up into a no-decision.

“When I came out (from behind the plate), I took a little look to see if anyone was coming. I didn’t want to hit anybody. It wasn’t the height that I could turn around. I had to try to catch it the way it was. I just dropped it, I guess,” said Hernandez, who said the mask on the ground had not gotten in his way.

“I would have had to dive for it,” said Gomez, who had moved back a few steps once the count had reached 0-and-2 on Crisp. “It was not really a high popup. It was kind of in-between.”

Gomez shook his head at the dropped popup and the snowball effect it generated.

“We always say pitchers shouldn’t catch popups. Why not? That’s silly,” said Gomez.

Stolen moments

There are times when Red Sox starter Josh Beckett concentrates so hard on the batter and making a quality pitch that he sometimes forgets to check baserunners, especially at second base.

In the first inning yesterday, with runners at first and second, Beckett didn’t even give Brian Roberts (at second base) a glance as he fired an outstanding curveball to Miguel Tejada for a strikeout, the first out of the inning.

Beckett barely peeked back at Roberts before his first pitch to the next hitter, Ramon Hernandez. As Beckett was getting his sign for the next pitch, Nick Markakis, the runner from first, was staring at Roberts to get an indication of what he might be thinking in terms of stealing third base.

First baseman Eric Hinske, meanwhile, was playing deep, way behind Markakis, so Markakis was able to get a good lead.

Beckett gave Roberts a long look this time, but Roberts had his mind set on stealing. So when Beckett threw the pitch to the plate, Roberts took off. Markakis, watching Roberts, took off himself when he saw his teammate headed for third.

The double steal ultimately netted the Orioles’ two runs and a 2-0 lead.

The throw to third from catcher Jason Varitek sailed off the glove of Kevin Youkilis and into shallow left, permitting Roberts to score and Markakis to take third. Markakis scored on a two-out single by Aubrey Huff.

He shorted himself

Manny Ramirez plays a very shallow left field.

He can steal liners over shortstop this way, turning seemingly certain hits into outs, and has done so many times. But the flip side is that because Ramirez is in so far, he has very little lateral range.

And that circumstance helped cost the Sox a run in the fifth yesterday.

Jay Payton hit a liner maybe 10 feet to Ramirez’s right, but Ramirez couldn’t get to the ball fast enough to cut it off on the bounce, so the ball went to the wall for a double on what would have been a routine single had the left fielder been playing in a normal position. Payton eventually scored an unearned run that put the Orioles on top, 3-0.

By the book

Baltimore’s Roberts had a textbook leadoff at-bat in opening the game with a walk.

The second baseman fell into a 1-and-2 hole to Beckett, but the Red Sox right-hander wasn’t able to put him away.

Roberts stayed alive by fouling off fastballs of 96 and 95 mph, took two pitches out of the strike zone and then fouled off a 95-mph heater. The ninth pitch of the at-bat was a fastball that was way inside, handing Roberts a well-earned walk.

In the fifth inning, Roberts had a similar at-bat against Kyle Snyder, again working a nine-pitch walk.

Just the way it goes

They say that everything evens out for hitters over the course of a season.

But sometimes things just aren’t fair in the here and now.

Over the course of three days the Sox’ J.D. Drew has been robbed of a pinch-hit two-run double on a line drive caught by a leaping shortstop, had a three-run homer taken away from and turned into an flyout and had a hit taken away from him on a nice play in the hole by Roberts, the second baseman.

And yesterday, in his first at-bat, it appeared as if Drew had beaten out a chopper up the middle, or at the very least that first baseman Kevin Millar’s foot was off the bag on a wide throw from Roberts, which likely would have been ruled an infield single had he been called safe. But first-base umpire Lance Barksdale called him out, ruling the throw had beaten him and that Millar had kept his foot on the bag, so it was just another fruitless at-bat for the struggling Drew, who was 7-for-50 heading into the game.

So what happened with the next hitter?

Youkilis cracked a high fly ball into the wind in left that was misplayed by Payton. The wind pushed the ball so far in toward the infield that Payton wasn’t able to catch up with it. The ball fell safely to the turf and Youkilis, who was 13 for his last 26, had a gift double.

skrasner@projo.com