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In one design, a digital isolator interface was powered from a 5-V supply while the bus connected to it operated with 3.3-V logic.

Nothing about the problem was particularly exotic. The signals simply didn’t reach the logic level required for the isolator to interpret them reliably as high. The information needed to catch the mistake was available in the schematic and in the component datasheet. Nobody, however, had compared the right parameters before the board was ordered.

This is the frustrating reality behind many late-stage design defects. They are not always caused by difficult calculations or obscure physical effects. Often, they are ordinary mistakes that nobody had time to look for, or relationships hidden across schematics, bills of materials (BOMs), datasheets and design documentation.