One of the finer details of being a head coach is knowing when to employ the challenge flag. The use of a challenge flag at the wrong time on a play that will not be overturned leads to the loss of an important timeout. However, the ability to trump a bad call or wrong placement of the ball is a key asset for a coach.
Luckily for the Chiefs, Andy Reid has turned into a good challenger. (Since it's the offseason, we have time to consider such things.)
For the newbie, an NFL head coach has two challenges per game to use before the two-minute warning. The challenge flag must be thrown before the next snap, and a team must have a timeout in order for a challenge to be used. The reason being that a team loses a timeout if the challenge is not successful (meaning the call was overturned). A coach receives a third challenge if the first two were successful.
It's hard to find any official figures on NFL coaching challenge stats, but Gary Myers noted the league-wide success rate for the 2009 season:
In the 2009 regular season, covering 256 games, there were a total of 328 reviews, 228 by coaches' challenges with 126 reversals. In the playoffs, 15 plays were reviewed with six reversals.
Andy Reid has issued a total of 90 challenges in his 15-year coaching career, and 44 of those have been overturned. That means his career success rate is 48.9 percent. That said, he was very successful last year, going 7 of 8 in his challenges.
The only miss for Reid last year came in during the Chiefs win over the Houston Texans in October. Alex Smith threw a short pass to Anthony Fasano on 2nd-and-7 for what Reid believed to be a touchdown. (Edit: Ball was on Houston's 7-yard line. Thanks to CT-Scott for the heads-up.) The refs ruled otherwise and the call was upheld.
Reid has had a few bad years in his tenure with the Philadelphia Eagles. He went 1 for 8, the inverse of his 2013 record, in 2005, and he whiffled completely in 2008 when he went 0 for 5.
For the sake of comparison, Bill Belichick has issued 95 challenges in his career and only 39 of them were overturned (41 percent). Mike Tomlin has won 24 of 44 career challenges (55 percent). John Fox is 40 for 110 in his career (36 percent), while Pete Carroll is 19 of 40 (47.5 percent).