BOSTON — He had been so good, so dominant, so — dare we say — perfect that it seemed incomprehensible at first.

Jonathan Papelbon had the ball and a two-run lead with three outs to go. Is there a better bet in baseball, a more sure thing?

Hardly. Papelbon had gone all of April without allowing a run. Heck, he nearly went the entire month without giving up a hit, allowing a measly two over the first 24 games the Sox played.

You see Papelbon trotting to the mound to the strains of “Wild Thing,” and you’re conditioned to think: Game over.

“Don’t you?” asked Brendan Donnelly of an inquiring reporter.

But this wasn’t the usual ending. In the span of just two hitters, Papelbon served to remind us all that no, he is not invincible, and no, he is not perfect.

He just seemed that way for all of April.

What seemed like a perfectly ordinary save, your basic punch-the-clock ninth, turned ugly in a hurry. A leadoff single to Bobby Crosby was followed in short order by a towering home run to right by Travis Buck.

An inning later, the A’s punched out two doubles off Donnelly and had themselves a 10-inning 5-4 win that they couldn’t have imagined.

Now what?

“We all get so used to him being almost perfect,” said manager Terry Francona after the night had been turned upside down on his team, “than when he’s not ...”

When he’s not, it’s stunning. The problem with being so good is it’s easy for people — fans, teammates, manager — to get spoiled.

“It’s very shocking for myself to see him blow a two-run lead,” said Coco Crisp.

No one was more shocked than Papelbon, particularly after he got ahead 0-and-2 on Buck.

“It was just one of those nights,” said Papelbon. “It’s not me taking things for granted or not working hard. Win some, lose some. It’s frustrating, obviously. We have the lead and don’t get the job done. My fastball command wasn’t good. My slider command wasn’t good. My split was just OK.”

The result, obviously, was not. The A’s looked feeble at the plate through the first eight innings. No one saw this coming, even after Crosby opened the inning with a single and certainly not after Papelbon threw the first two pitches of the at-bat to Buck for strikes.

“When you get a guy 0-and-2,” said Papelbon, “my mindset is he should be out as long as I execute my pitches. I just didn’t execute that pitch.”

That pitch, the one in question, was an elevated fastball that, by his own admission, Papelbon didn’t elevate enough.

“It was right over the heart of the plate,” he said.

So we’re not in April anymore. Just ask Papelbon and Donnelly.

Neither had allowed a run in the first month of the Red Sox season. But April is gone, and after the ninth and tenth innings last night, so are the relievers’ scoreless streaks.

Papelbon had eneded the first month with a perfect 0.00 ERA. But here he was last night, needing 35 pitches to get three outs.

Donnelly, who had been equally spotless in April, gave up doubles to the first two hitters — Mike Piazza and Dan Johnson — in the tough-to-take setback to the punchless A’s, who came into the night 12th in runs scored and 13th in batting average in the American League.

If you thought Papelbon might go the entire season without being scored upon, you’re forgiven. There were times when that didn’t seem entirely out of the question.

Clearly energized by his return to the role he longed for, Papelbon had held opposing hitters to just two hits in 30 at-bats.

His 95 mph fastball looked all the more fearsome, thanks to an adjustment in his delivery that made it even more difficult for hitters to track.

Indeed, there were times it seemed patently unfair for hitters to face Papelbon. If his death stare didn’t intimidate you, his stuff surely would.

But not last night.

“What can I say?” shrugged Papelbon. “You ain’t gonna win ’em all. That’s the beautiful thing about baseball.”

Beauty being in the eyes of the beholders, of course. For the Red Sox, it got unexpectedly ugly in a hurry last night, and no one saw that coming.

smcadam@projo.com