Five minutes earlier, Grant Gregory, after almost getting sacked for a huge loss, put Kansas State (3-2, 1-0 Big 12) ahead 24-17 with a 54-yard touchdown pass to Brandon Banks. It was the second TD pass of the fourth quarter for Gregory, a surprise starter.
Overtime seemed certain after Arnaud connected with Jake Williams, who beat two defenders in the back of the end zone. But Lamur, a 6-foot-4 defensive back, got a hand on the extra point attempt, stunning Iowa State fans who were still celebrating the TD.
Gregory also threw a tying 16-yard TD pass to Lamark Brown early in the fourth period.
Iowa State (3-2, 0-1) led 17-10 in the third period after Arnaud's 3-yard TD run capped a 12-play, 80-yard drive.
The game was played in Kansas City's Arrowhead Stadium in the first of a two-year contract between the schools, and drew a disappointing crowd of 40,000. Next year, the Wildcats will be the "home" team.
Arnaud, who threw for 164 yards and rushed for 87, kept Iowa State's final drive alive with a 22-yard completion to Marquis Hamilton on fourth-and-5.
After Guidry blocked the extra point, Iowa State vainly tried an onside kick.
On Kansas State's final touchdown, Banks, who tied an NCAA record the week before with two touchdowns on kickoff returns, shook loose from a would-be tackler as he hauled in the ball and sprinted for the end zone. Gregory was almost sacked on the play and made the throw to his left while running to his right.
Iowa State's Alexander Robinson, the second-leading rusher in the Big 12, did not return after making a 37-yard run to set up Grant Mahoney's 32-yard field goal in the second quarter. Robinson had been nursing a pulled groin muscle since playing against Army the week before.
Gregory, a transfer from South Florida given a sixth year of eligibility, made his first start for the Wildcats, He replaced Carson Coffman, and supplied a spark missing throughout most of the nonconference season. He and Daniel Thomas alternated the first few plays, then Gregory took over on an 89-yard drive he capped with a 2-yard run.
Gregory was 16 of 23 for 206 yards and two touchdowns, and Thomas carried 25 times for 96 yards.
The Cyclones took a 10-7 lead late in the first half when tight end Derrick Catlett got wide open over the middle for a 22-yard TD pass from Arnaud.
In the final few seconds of the half, Iowa State appeared to stop Kansas State on the Cyclones 32. But pass interference on Fred Garrin gave the Wildcats the ball on the 23. With 5 seconds left, Josh Cherry, who was 1 for 6 in field goals, managed to get a wobbly knuckleball to sail true on a 39-yarder.
After taking a 17-10 lead, the Cyclones were on the move again when Catlett fumbled following a 12-yard reception and Tysyn Hartman jumped onto the ball for the Wildcats.
Aided by a 15-yard personal foul penalty against Iowa State, the Wildcats went 36 yards in seven plays to tie it 17-17 on Gregory's 16-yard pass to Brown, who was uncovered in the end zone after his defender fell down.