Here's the rule I've learned after years of instant replay in the NFL:
By throwing that red flag, you can turn ANYTHING into an incomplete pass.
Brady fumbles. Belichick goes to his sock and pulls out the challenge flag. Incomplete pass.
Polamalu intercepts. Dungy grasps at straws desperately. Incomplete pass.
I've seen this happen more than once. The two examples I just mentioned are the most absurd. But completions and interceptions get overruled all the time for one simple reason. Using today's NFL rules, a complete pass frequently LOOKS like an incomplete pass when viewed in slow motion.
Shift everything down to 1/8 speed and the refs see the tiniest movements of the ball in the receiver's hands. They say he was "bobbling it" and never established control. A perfectly good completion in real-time can look like a turbulent froth of primordial ooze when viewed under the microscope.
I play ultimate frisbee whenever I can. It's a lot like football in that your job is to catch the "ball" before it hits the ground. It's a classic American game of of "People vs. Gravity". Like the NFL, some ultimate players make some AMAZING catches: laying out for a disc that's about to hit the grass or go out of bounds. These feats of athleticism should be rewarded, regardless of weather the disc grazed a blade of grass as the player was finishing his catch.
For the NFL, I say "loosen up the completion rules". If it looks like a completion in real-time, it probably was a completion. Give these guys credit for the outstanding plays they are making, and play on!