| Picking only by pin count | Ignores current, voltage, contact size, environment, tooling |
| Ignoring backshells | No strain relief, poor EMI, poor sealing |
| Ignoring tooling | Design may be impossible to build correctly |
| Mixing signal and power casually | Noise, safety, creepage, heating, service confusion |
| Bad shielding (pigtail) termination | Pigtails are inductive — raise impedance, radiate |
| No strain relief | Conductors fatigue and fail at the termination |
| No keying | Similar connectors get swapped; expensive mis-mates |
| Poor service access | Technicians damage connectors during maintenance |
| Hobby connectors outside their suitable environment | No sealing, weak latch, unknown vibration life |
| Not checking mating cycles | Test/service ports wear out early |
| Not checking lead time | Schedule failure despite a good design |
| Not documenting pinouts | Harnesses become tribal knowledge |
| Wrong powered gender | Exposed live pins → shock/short risk |
| No dust caps | Dirt/water/pin damage on unmated ports |
| Using spare pins randomly | Future maintainers inherit chaos |
| No cable labels | Debugging becomes painful |
| No mating connector in BOM | Procurement buys half an interface |
| No torque spec | Sealing and anti-vibration features compromised |
| Confusing IP67 with IP68 | IP67 ≈ 1 m / 30 min; IP68 = stated depth/duration — not interchangeable |
| Undersized wire in wire seal | Cavity leaks; whole connector seal defeated |
| No ground-first mating sequence where needed | Power-before-ground → latch-up, ground bounce, resets |
| Substituting "equivalent" parts without qualification | Plating/insert/thread/geometry can differ; not drop-in |
