Jared Allen is never one to muffle his emotions, but it didn't require a body language expert or lip reader to discern his mood as the Vikings defense changed ends of the field before the fourth quarter Sunday.

Helmet in hand and his face bright red, Allen screamed at anyone and everyone in his vicinity as he stalked the length of the field. The Vikings once again were on the ropes, and their Pro Bowl defensive end appeared to be in meltdown mode.

"In my opinion, it's not OK to let them score," Allen said later. "It's not OK to let these things snowball again. It's not OK that we've lost three games in the second half. It's not OK that we've given up a 10-point lead, a 17-point lead and now a 20-point lead. That's unacceptable. That's not OK. That was [the] message I was trying to get across."

And yet it happened again, a third consecutive second-half collapse in a 26-23 overtime loss to the Detroit Lions. And once again the Vikings defensive players were at a loss for words trying to explain how things fell apart after another dominating first-half performance.

Even a broken record is tired of this song.

There's plenty of blame to share in the latest debacle, from inept offense to poor coaching decisions. The Vikings defense didn't hold up its end of the bargain either.

The discrepancy in the defense's performance from one half to the next is confounding. Just like a week ago against Tampa Bay, the Vikings pitched a shutout the first half and made it look easy.

They kept constant heat on Detroit quarterback Matthew Stafford, bottled up the run and left the Lions completely discombobulated. Detroit managed only four first downs and 50 total yards in the first half. The Lions didn't allow a sack their first two games. The Vikings sacked Stafford three times before halftime and pressured him a handful more times.

But then halftime came and everything got turned upside down. The Lions used a quick-hitting passing game to counter the pass rush. They feasted on short and intermediate passes over the middle, particularly tight end Brandon Pettigrew, who was targeted 10 times in the second half.

Stafford got hot, Calvin Johnson was Calvin Johnson, and the Vikings defense couldn't get off the field on third down. The Lions running game sputtered, but it didn't matter.

Stafford completed 11 of 15 passes for 189 yards and one touchdown -- in the third quarter. He was 22-for-30 passing for 314 yards in the second half.

Johnson made two spectacular catches, including a Willie Mays over-the-shoulder grab that set up the game-winning field goal.

The Vikings' inability to do anything on offense didn't help matters, but the defense simply didn't make enough plays or adjustments in the second half to stem the tide.

"You couldn't possibly think we're messing up that bad to give up 20-something points in the second half," defensive tackle Kevin Williams said. "They're not throwing anything major downfield. Just lollipops on some of those throws. I know Calvin is big, but good Lord we've got to do something."

The defense finally came to life at the end of regulation after back-to-back false starts on Lions left tackle Jeff Backus backed Detroit up to the 10-yard line. Allen followed with a sack and then nearly tackled Jahvid Best in the end zone for a safety.

But even that wasn't enough. A foolish unnecessary-roughness penalty on Kenny Onatolu on the punt return all but squashed any hope of winning in regulation.

Allen gave a long, exaggerated sigh as he sorted through the mess afterward. "Through adversity comes great things," he said.

His teammates likewise sounded out of answers in the postgame locker room. Linebacker Chad Greenway insisted "nothing changed" in the second half in terms of major adjustments other than the Lions made plays and the Vikings didn't. Nobody questioned effort this week.

Allen even suggested a headline for the latest collapse: "I don't know."

"It would be one thing if it was a lack of effort or if there was an issue," he said. "It ain't a lack of effort, and we've got talented guys. You see what happens when we fire on all cylinders. People can't move the ball on us. Something happens."

That something left Allen fuming at the end of another wasted opportunity.