An error is an error, and it needs to be corrected no matter whom it benefits.
To knowingly remain silent about a timecard error that benefits you is no different from theft.
In payroll, we used to see overpayments all the time. From system problems, to incorrect timecards, to incorrect work codes, to incorrect rates, etc. From our perspective, very few employees ever mentioned the error. If we caught it or the center asked us for a correction, it was like pulling teeth to recover the overpayment. Some state laws permit deducting overpayments from future paychecks but some state laws required us to send a letter asking the employee for permission to take back the funds. (Guess how many of those letters were ever returned??) If it was a larger amount, we were more than willing to split the repayment over several weeks to minimize the impact.
I could understand how an employee may not notice a small overpayment. The way the rates work for different types of work can be very confusing and if you work multiple PT jobs in a day, it gets tricky. But when you see a sleeper driver getting double paid for several months, you have to wonder why he didn't feel morally obligated to mention it. It wasn't small dollar amounts. It was an extra $1500 - $1800 per week. (Shockingly, there was no comment when we caught the problem and stopped the double payments.)
Or the PT employee who attempted a FT driving job. He apparently didn't make his 30 days and returned to his former PT job. But HR didn't change him back to PT. So, he kept getting paid as a FT driver, moving through the seniority steps, getting the driver wage increases, getting 8 hour holiday/personal days. HR finally corrected him about a year later and backdated the change to when he returned to the PT work. This meant that we were supposed to look at all of his timecards for the year (which clearly showed he was working his PT job) and calculate what he should have been paid at the PT rate compared to what he was paid at the FT rate. In addition, all of his holidays, vacations, etc should have been paid at fewer hours. It was a difference of thousands of dollars. Should he have known that his rate should have dropped back down to his PT rate when he returned to his PT job? Yes. Did HR drop the ball by not correcting his job classification in the system? Yes. Fault lay with both sides. I'm not sure how that one worked out but I think all parties agreed that it was ridiculous to ask him to repay that kind of money.
Mistakes happen. UPS has the responsibility to issue payments correctly and take swift action when corrections are needed. This is why payroll issues manual checks to correct errors on a daily basis instead of waiting for the next paycheck. At the same time, I would hope that anyone who received additional (unearned) pay in error would have the personal ethics to report it.