The eternally curious Tailsteak has written about how he always picks up pennies off the sidewalk. He's run a cost-benefit analysis and determined that it's better on average to pick up a penny than to pass it by. His mistake lies nowhere in the analysis itself; it's pretty much correct. His mistake is performing the analysis in the first place.
Pennies, you see, are easily the subject of scope insensitivity. When we come across a penny, we don't think, "Hey, that's something worth 0.05% of what I wish I had come across. I could buy a 25th of a gumball, a mouthful of an unhealthy carbonated beverage, a couple of seconds of footage on DVD, or enough gasoline to go a tenth of a mile." We think, "Hey, that's money," and we grab it.
The thing is, it's difficult to comprehend how little a penny is worth—we don't really have a separate concept for "mild happiness for a couple of seconds"—and we're likely to take risks that far outweigh the benefits. We don't think of bending over to pick up a penny as being a risky endeavour, but it's a penny. How much risk does it take to outweigh a penny? Surely the risk of "something unforeseen" easily does the job. Are you 99.999999% sure that picking up that penny won't kill you? You need a reason for every 9 (if you're ambivalent between using seven 9s and using nine 9s, you should use seven; the number of 9s is never arbitrary), and by the time you come up with eight reasons to pick up the penny, you'll have wasted several cents' worth of time. If you can reduce the probability of harm that far, I applaud you.
Of course, penny-grabbing doesn't have to involve actual pennies. Suppose that President Kodos of the Unified States of Somewhere (population 300 million) uses the word "idiot" in an important speech, causing the average citizen to scowl and ponder for one minute. Now, if a penny can buy you five seconds of happiness, and scowling and pondering brings the same amount of unhappiness, then that's twelve cents for every citizen, or 36 million dollars, of damage that Kodos just caused. Arguably, that's the value of a couple of human lives. As you can see, Kodos' decisions are extremely important. In this case, penny-grabbing would consist of anything less than trading precious seconds for precious human lives—if Kodos finds that he can save one life simply by going a few minutes out of his way, he should ignore it. (Photo ops and personal apologies are out of the question.) But keep in mind, of course, that avoiding saving someone's life because you have something better to do isn't rational unless you actually plan to do something better.