Funny story.
A customer came back to me with problems they had with a production batch of a design I completed few years ago. Suddenly large proportion of the batch was not working properly.
Years of working in software engineering have taught me to always first to look for my own screw-ups, since that is usually the most common cause for any problems  So calls like that always worry me. Did I miss something?
The device is a low power contamination detector that shines a light through a sample and measures the response. Quick tests showed the photodetector side was not working properly. The circuit is a standard transimpedance amplifier straight from datasheet. In absence of photo-current through the diode it works as a unity gain buffer, but on the affected device the output was less that than the non-inverting input. How is that even possible? Transplanting the questionable opamp onto test PCB showed that it is actually working, but the input bias current specs is not met. The behavior was consistent with parasitic current between VDD and inverting input. The amplifier used in the device is LTC6244, that has pA bias currents, but I was observing currents in the range of uA. Having experience with low power circuits in the past, I started to suspect PCB surface leakage. That was quickly ruled out, since VDD and OPA1- are on opposite sides of the package and there are traces separating them. ESD damage? Plausible, but there was more than one board with EXACTLY the same behaviour. Replacing the funny amplifier with a new one fixed all of them. When the number of boards showing the same strange behavior became 10 out 10, I had to rule out ESD as well. If the pin is not just shorted to VDD or VSS, what are the chances of developing identical leakage on 10 boards by ESD?

Chips looked ok, marking looked legit. But, I saw that lead frame protrusions were in different locations compared to the known proper chip. But ok, my proper chips were purchased 3 years ago. Maybe the company has made a chip revision or something? And then I cranked up zoom on my microscope to the maximum. Whoa! I’ve never seen that on any other chips, ever! And it was not just one of the like that. They were all like that! Bond wires sticking out?