The Horned Frogs couldn't generate any offense and their sloppy special teams unit had problems holding onto the football in a heavy rain.
Despite all the problems, the 15th-ranked TCU kept on winning and paid the Mustangs back for a difficult loss last season.
Casey Pachall threw two touchdown passes, Jason Verrett had two interceptions and TCU extended its FBS-best winning streak to 12 games by beating SMU 24-16 on Saturday night.
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"Whoever didn't screw it up was going to win this game," TCU coach Gary Patterson said. "I'm not sure we didn't screw it up more and just didn't get lucky."
The Horned Frogs (4-0) managed just three points and 27 yards of offense after halftime.
TCU botched a snap on a field goal attempt in the third quarter. In the fourth, punter Ethan Perry lost the ball twice before he was able to kick, giving the Mustangs a chance to get back in the game.
The Horned Frogs intercepted Garrett Gilbert five times, including Verrett's pick of a desperation heave on the final play of the game. That was enough to avenge a 40-33 overtime setback to the Mustangs a year ago, which was TCU's last loss.
"Our defense just played great," TCU sophomore linebacker Joel Hasley said. "They gave us what we thought they were going to give us and we went out there and played good."
TCU has won 11 of 13 over SMU and regained the Iron Skillet trophy, which goes to the winner of the Dallas-Fort Worth rivalry.
Horned Frogs running back Matthew Tucker scored two touchdowns in the first half that capped drives of less than 10 yards.
The game was played in a heavy rainstorm. The rain picked up in intensity around kickoff and didn't let up until the game ended.
"I knew the defense was going to have to step up," said TCU safety Elisha Olabode, who had an interception to set up a touchdown. "It was a rainy game, a defensive game."
The Horned Frogs jumped out to a 21-10 halftime lead and held on despite a season-low 156 yards of offense.
Zach Line had a 21-yard touchdown run in the second quarter for the Mustangs, which lost for the second time this season to a state rival from the Big 12. Baylor beat SMU 59-24 in the season opener on Sept. 2.
Gilbert, a highly-touted recruit who transferred from Texas in the offseason, has thrown nine interceptions this season. He threw four in the second half and the Mustangs turned it over six times overall
"There were a couple he shouldn't have thrown," SMU coach June Jones said. "That's part of being a quarterback. You have to fight through and get better."
The Horned Frogs committed 13 penalties for 140 yards, including seven personal foul calls.
Punter Ethan Perry couldn't field a low snap, giving SMU the ball at the TCU 35. Gilbert hooked up with Der'rikk Thompson for a touchdown on the next play, cutting the deficit to 24-16 with 2:08 left.
The Mustangs had one last chance when they got the ball at their own 33 with 1:06 left. They moved the ball 9 yards before Verrett intercepted Gilbert's Hail Mary attempt.
Pachall, who came into game leading all FBS quarterbacks in passing efficiency, was 10 of 26 for 107 yards. Tucker rushed for 56 yards and Josh Boyce added 42 yards receiving for the Horned Frogs.
TCU's only points in the second half came on Jaden Oberkrom's 43-yard field goal early in the fourth.
Gilbert was of 15 of 40 for 190 yards, but his receivers dropped at least six passes.
"Our receivers have to step up and make those plays," Jones said.
TCU safety Chris Hackett recovered Line's fumble at the SMU 41 to set up Pachall's 10-yard touchdown pass to LaDarius Brown.
Olabode intercepted Gilbert and returned it 51 yards down to the 8. Three plays later, Pachall found a wide open Tucker on a 2-yard scoring toss.
After picking up only 4 yards of offense in the first quarter, the Mustangs put together their only substantial drive.
Gilbert hit 4 of 5 passes for 56 yards and Line finished off an 86-yard drive with his touchdown. The big back broke three TCU tackles and then skipped over another on the way to his third score of the season.
Skye Dawson returned a punt 64 yards down to the 4, and Tucker scored on the next play to extend TCU's advantage to 21-7 with 2:11 left in the first half.
Chase Hover kicked a 29-yard field goal just before halftime to bring SMU within 21-10.
The rain caused problems for both teams as TCU head coach Gary Patterson braved the elements without any kind of rain jacket or slicker.
"It's just water," Patterson joked. "I can't coach constricted."
SMU receivers had three drops on its first two possessions of the game.
Tucker slipped twice making cuts on consecutive running attempts, and TCU's holder couldn't handle a snap on a field goal attempt in the third quarter.
The Mustangs had a scoring chance early in the fourth when Perry mishandled a snap, and SMU's Austin Fuller picked up the loose ball at the 1. After Line was stuffed and Gilbert was sacked, the Mustangs threw two incompletions into the end zone to squander the opportunity.