Mother nature is not fair and stuff can fail any way that it pleases. There is always a danger that you'll never fix the situation after assuming that the failure must be [...] or cannot be [...], however, one should play the percentages and avoid chasing the 0.01% probability first.

Case in point: Customer called saying that Comcast sent them a new gateway, self installed, and the network is now down. As a quick, over the phone fix, we suggested reverting to the original gateway, but Comcast would not go along with that and they ended up putting the new gateway in bridged mode (correct plan of attack for this installation). Things were working. She then left town for summer vacation and he called three days later with "network down", suddenly while he was typing. Our first assumption centered on DHCP ranges, renewals, this or that had been reset or not reset -- the usual network fumbles. However, when we arrived we discovered cables not fully seated.