What's the Biggest RJ45 Selection Mistake You've Seen?

I've been collecting common RJ45 connector mistakes for a guide, and these five come up repeatedly:

1. Mismatched magnetics
2. Under-spec'd PoE contacts
3. Wrong IP rating
4. Insufficient mating cycles
5. Missing protection circuits

What's the most painful connector mistake you've encountered (or made)?

Full guide with fixes here: https://voohuelectronic-ux.github.io/voohu-rj45-guide/resources/rj45-selection-mistakes.html