More specifically, for a digital signal, what does "working, but not up to spec" look like? Occasional packet loss? Corrupted data that is masked by ECC?
"working, but not up to spec" can mean "works today, stops working tomorrow when subjected to a bit more nearby electrical noise". 10GbaseT links frequently can't negotiate down, so your link can just stop working whenever.
Can also mean: Works with devices A and B, stops working with devices A and C.