Observed failure
An aftermarket engine control unit intermittently lost USB communication with a connected PC. The failures appeared associated with ignition interference, but the actual root cause was unknown.
Investigation
I compared the physical USB connector to the schematic symbol and noticed that the connector had six pins while the schematic represented only five. A previous five-pin connector design had been reused, leaving the sixth pin—the USB shield connection—unrepresented.
Correction and result
Adding the missing shield connection resolved the observed communication issue. The key lesson was to verify the physical connector pinout against the library symbol, especially when revising a design around a different connector.