In the top of the 5th last night, the Tigers put runners at 1B and 2B with nobody out and Don Kelly tried to lay down a sacrifice bunt. He popped it up and Daric Barton, endeavoring to be extra-smart, let it drop with the intention of firing to 3B for one force play, with Inge then whipping it over to 2B for a DP. Trouble is, the ball landed near the 1B line and then kicked sharply foul. Oops.
The seed idea itself was good. On a popped up bunt, even with two runners on the infield fly rule is generally not called because the ball is not hit high enough to apply the rule (which is intended for when an infielder can camp under a pop-up). So the chance to take advantage of multiple force plays and frozen base runners exists.
However, you absolutely, unequivocally should never let a pop up drop when it is anywhere near the foul line. That concept is reserved for balls popped up near the mound, not near a base line. Had the ball been popped up back to McCarthy, it would have been absolutely a reasonable play for McCarthy to let it drop and then fire to 3B. This gets the lead runner easily -- same result as catching the pop-up -- with at least a chance to complete a DP. Had the ball been popped up to Barton's right, it would have been a defensible play. As it was, the play was simply a clear blunder.
Now to a situation that didn't come up last night but is similar: Only a runner at 1B and a sacrifice bunt is popped up, clearly fair, e.g. back to the pitcher. What's the right play? Very few people get this one right. Think about it before you jump, and then read on...
The correct play is to let the ball drop and then immediately throw to 1B. Why? Because the runner you're less likely to get out for the DP is the batter, because the batter can run immediately and doesn't have to wait to see if the ball is caught or not. If you catch the pop-up, you will get one out and the runner will get back. But if you let it drop and throw to 2B, you will easily get the runner while the batter is likely to beat the relay to 1B.
However, if you let it drop and immediately fire to 1B, the 1Bman can tag 1B to retire the batter and then has the runner in an easy rundown. This is because the runner has to hold until the ball bounces, and it takes only an instant to field the bounce and fire to 1B. The runner is going to be about 20 feet from 1B when the batter is retired. 1-3-6, pretty much a sure-fire DP. Pretty nifty, eh?