The Bills came due Sunday afternoon, and the Bears paid a heavy price.
Mental mistakes and aggressive miscues finally caught up with the Bears, and Buffalo made them pay for every indiscretion.
After taking a 7-6 lead with 12 seconds left in the first half, the Bills went on to roll 35-20 over the previously undefeated Bears.
A Rich Stadium sellout crowd of 80,366 roared as the Bears committed 12 penalties for 89 yards-10 of them in the first half.
The loss left the Bears (4-1) in a first-place tie with the Detroit Lions in the NFC Central Division.
"We got our butts beat, that's all you can say about it," said coach Mike Ditka. "We had a number of chances and we seemed to self-destruct every chance we had. I give them all the credit in the world."
The defending AFC champion Bills (5-0) have won 12 games in a row, including the playoffs, at home. Their last loss at Rich Stadium was in November 1989.
"They're a fine football team; they taught us a little lesson," Ditka said. "Hopefully, when you're in football, you learn from the lessons that are taught to you, and you carry on from there."
The Bears host the Washington Redskins next Sunday at Soldier Field. The Redskins (4-0) meet the Philadelphia Eagles Monday night.
"We're still a pretty good football team. We just have to come back to the drawing board and play the kind of football we're capable of playing," said Ditka, whose teams are 23-3 in the month of September since 1985.
The Bears controlled the ball an incredible 22 minutes 2 seconds, compared to the Bills' 7:58, in the first half. Yet the Bears managed to cash in with only a pair of field goals, from 28 and 42 yards, by Kevin Butler.
The Bills got on the board with a 33-yard touchdown pass from Jim Kelly (19 of 29 for 303 yards and 3 TDs) to Al Edwards with 12 seconds left in the half.
"The touchdown before halftime was a big pickup," said Bills coach Marv Levy. "We had to go 85 yards to get it, so it was an earned touchdown, not a fortunate touchdown."
Edwards was isolated on cornerback Donnell Woolford when the Bears' secondary was supposed to be in four-deep prevent coverage.
"The guy should have never been in that situation to get beat," Ditka said. "We broke the defense and misunderstood the way it was supposed to be played.
"We had enough mistakes out there to last a season. Hopefully we can correct a few of them."
Rookie left tackle Stan Thomas was called for two false starts and an illegal hand to the face trying to block All-Pro Bruce Smith. The Bears also had two touchdowns called back because of penalties.
With the game scoreless in the first quarter, Kelly's pass was intercepted by free safety Mark Carrier. He returned the theft 43 yards into the end zone. But the play was called back because second-year defensive end Tim Ryan, starting in place of injured Trace Armstrong, had jumped offsides.
"I was just thinking about getting off the ball," said Ryan. "There was no excuse for that. Stupid play. My fault. That's all there is to it."
"We had a lot of penalties and it cost us two touchdowns," Ditka said.
The second TD called back was a 17-yard pass from Jim Harbaugh (17 of 35 for 203 yards with one interception) to Neal Anderson in the second quarter. Rookie fullback Darren Lewis was called for offensive interference and the Bears had to settle for Butler's 42-yard field goal and a 6-0 lead.
The Bills' second-half scoring onslaught was typical of their league-leading offense.
Buffalo took the second-half kickoff and went on a 12-play, 91-yard drive that ended with fullback Carwell Gardner diving in from the 1 for a score. Gardner fumbled the ball into the end zone. Since he recovered his own fumble, the score was allowed and the Bills led 14-6.
The drive started with runs of 12, 9 and 19 yards by Thurman Thomas, who got 90 of his 117 rushing yards after intermission. Thomas also caught three passes for 40 yards.
"We got them on the 9-yard line, but they ran the ball right down our throats," said Ditka. "So we have to have a little more mental toughness." Buffalo ran up 421 yards total offense, 298 in the second half.
"When you score seven points after being shut down most of the day, that's pretty big," Bears linebacker Ron Rivera said of the Bills touchdown that closed out the first half.
"They came out (in the second half) and I think they caught us a little bit off-balance with some of the things they did. We didn't anticipate it."
"They ended the half by throwing the ball the way they did," Rivera said. "We thought that would be pretty much their mode of operation (in the second half)."
"I just think they were trying to get the ball out to where they could work with it in the second half," Ditka said. "They hit a gold mine. Whatever (adjustment) they did, they did well. That's for sure.
"This game is a 60-minute game. It will always be a 60-minute game. And you must play it for 60 minutes. You must play it right.
"I can't tell you how much miscommunication and errors we had in the football game. You saw all the penalties. That was enough in itself to make you sick."
Kelly hooked up with tight end Butch Rolle on a 2-yard TD pass with 4:46 left in the third quarter. It was Rolle's 10th consecutive catch that resulted in a TD.
The Bears drove to the Bills' 1 early in the fourth quarter before being stopped on downs. An attempted third-down pass to Ron Mattes on a tackle-eligible play was knocked down by cornerback Nate Odomes. Then Odomes stepped up to stuff Anderson on fourth down.
Harbaugh was only 6 of 16 passing in the second half.
"In the second half I couldn't get any time to throw," Harbaugh said. "I mean, every time I went to throw, there was a linebacker running through. I could never get my feet set. So we missed about five straight passes on two straight drives."
The Bears pulled to within 21-13 on a 2-yard Anderson run early in the fourth quarter.
But the Bills retaliated 22 seconds later with a 77-yard bomb from Kelly to 35-year-old James Lofton to make the score 28-13. Thomas' 10-yard run made it 35-13 to seal the rout.
Backup quarterback Peter Tom Willis came in to direct a 66-yard scoring drive that resulted in a 1-yard run by Mark Green in the final minute. Willis was 5 of 7 for 66 yards in the march.
"They spanked us; I don't know how else to analyze it," Anderson said. "They beat us every way possible in the second half. They outplayed us."