On 15 July, there was a slip in my mailbox from the letter carrier, declaring that 71¢ postage were due on an item, which could be redeemed and retrieved at the post office after 09:00 on the next day. I was explicitly named on the slip.
Had this been an item that I'd allegedly sent without sufficient postage then, instead of my just receiving a slip, the item would have been physically returned, with a demand for more postage; so it was something sent to me.
USPS rates for First-Class mail are 49¢ for the first ounce, and 22¢ for each ounce thereafter. So, if someone were to misjudge the weight of an item, then it would be expected to have some integer-multiple of 22¢ too little (or too much) postage. To be 71¢ short, it would most likely have been dropped in the mails unstamped, or had all of its stamps stripped by postal machinery; in the latter case, one expects the stripping to occur sooner rather than later.
The most likely thing would be that this item were without stamps very early in process. And, in that case, it would have been delivered to the return address, with a demand for more postage, if there were a return address; so I guessed that there weren't. That had me curious.
Very shortly after 09:00 on 16 July, I was at the post office, with the slip. But the postal clerk was unable to find the item, and the carrier was not available. (He or she was probably already out, making deliveries.) The clerk insisted that she would take care of the postage due — I suspect that there were no provision for me to pay postage due on a lost item! — and have the carrier deliver the item.
However, it was not in my box on 17 July, nor on 18 July; it would seem still to be mislaid. So I'm left to conjecture.
On 10 September, there was another slip in my box, declaring 71¢ postage due. While it might have been for yet another item, my guess was that it were for the same piece, having resurfaced. I had reason to go to the post office anyway, as some registered mail was there waiting for my signature.
When I attempted to pay for and collect the mail with postage due, it was again declared to be lost. The fellow behind the counter angrily resented my angry resentment, and I demanded to speak to his manager. The manager found the mail, which was lost as one more aspect of not following normal procedures.
The item was, as it happened, indeed the same item, and something that I had mailed. My scale had said that it were one ounce; apparently theirs said that it were two. So it should either have been sent on to the addressee, with a demand for 22¢ more postage, or returned to my box with that demand, instead of my having been summoned to the post office with a demand for 71¢.
Instead of arguing about 41¢; I just decided to take the thing home, and not to resend it. There is no love lost between the intended recipient and me; and I consider him to be the primary victim of the USPS in this case.
 Some day, I plan to invest in a one-pound weight of the quality used by Bureaus of Weights and Measures, and visit various post offices, testing the scales of their automated dispensers. My guess is that almost every one will overstated the weight. I don't expect that I'll have an opportunity to test the other scales, but I'd bet the innacuracies to be coördinated.