Back in the day, we had an "in house" built TV transmitter remote control unit (one of two) in for a modification for one of the programme inputs.
In the "guts" we used a 4000 series monostable IC (too long ago, so I don't remember the number), which was used to obtain a relatively long time delay.
For some reason, whilst working on the equipment, I removed this IC & carefully placed it to one side, where it promptly fell on the floor, where try as I might, I couldn't find it.
"Bummer!" said I, but consoling myself with the fact that we had plenty in stock, I grabbed one from the parts store & continued with the "mod".
When I finished & tested the unit, it didn't work correctly, as the mono always timed out early.
Assuming it was one "rogue" device, I grabbed some more, all of which had the same problem, while another mono of the same type from another part of the unit worked correctly.
After much digging through data books (No Internet back then), it turned out that people had been having problems with erratic operation when using short time delays, so the problem was fixed by optimising the device for such intervals, to the detriment of long ones.
The problem was that they kept the original number for these modified ones & allocated the original device a new number.
We bought a bunch of the new number, which worked as before, changed the remote control's documentation to reflect this, then when the second unit was taken out of service for the original "mod", changed the ICs in that one too.
OK for us, with two units---imagine if we had thousands of them in the field & in production!