NFL challenge rules: What can a coach challenge (and not)?

This might be news to you, but referees aren't perfect. Like people in any and every profession, occasionally, they get things wrong from time to time. Unfortunately for NFL referees, these mistakes come when millions of eyes are watching them. But the National Football League has a method in place for combating questionable calls -- it's called the challenge flag.

In the NFL, head coaches have the ability the launch their red flag onto the field -- signaling that they would like to challenge the previous play. Some coaches have a good track record when it comes to this. Others, like Mike Tomlin of the Pittsburgh Steelers, have difficulty coming away victorious more often than not when they let the challenge flag fly.

It's important for head coaches to make wise decisions when it comes to whether or not to use their challenge flag. A failed challenge results in a loss of timeout, and two failed challenges mean that their team will not have a chance to challenge a call for the rest of the game.

Scoring plays are automatically reviewed by officials. It's also up to the judgment of referees to signal an officials timeout inside the two-minute mark if something deserves a second look. So what types of plays can coaches challenge, and what calls on the field cannot?

What can NFL coaches challenge?

In the past, scoring plays were commonly reviewed, but this is now left up to the discretion of the officials. Anything that is questionable (whether it's a player crossing the goal line, maintaining position, keeping their feet in bounds, etc.) is automatically reviewed upon a score. Nowadays, challenge flags are used more frequently in other situations.

Challenging whether or not a fumble has occurred is common in the NFL. This can be tricky, as these plays often leave room for subjectiveness in trying to determine where possession of the football begins. In addition to fumbles, coaches often challenge plays that occur along the sideline. Did a player get both feet in bounds before stepping out or did he maintain position? Challenges are often used in situations like these.

Head coaches can also challenge plays that occur near the goal line. For instance, if they think a ball carrier broke the plane of the endzone, but it was not ruled a touchdown on the field and there wasn't an officials timeout to review it. They may also challenge whether or not a knee was down if a player fumbled before crossing the goal line or stepped out of bounds prior to the score.

What can't be challenged?

Scoring plays and those inside the two-minute mark are automatically reviewed and cannot be challenged. Likewise, interceptions are also automatically examined by officials. Unfortunately, coaches cannot challenge penalties -- no matter how much we might disagree with the call.