Recently, I spent an evening with my ISP working on a modification to the wireless system. We couldn't figure out why the modification wasn't working. The wired network segments didn't function as expected.

He was so sure that his cable crimps were correct that he didn't re-check them until after I'd badgered him about it a few times. And even then, when he checked them, they "looked fine to him". Finally, I dug out a "known good" cable of sufficient length and sure enough, everything started working. Only after being shown that a different cable solved the problem did he notice that there was in fact a subtle problem with the crimps.

Moral of the story: Never trust cables you crimped yourself. If you have communications problems with handmade cables, always suspect the cables first. If you have communications problems with store-bought cables, suspect those first, too. Always swap out cables and connectors with known-good ones as the first diagnostic step. This goes for just about anything: Networks, disk drives, serial communications, telephones, audio, video...

I've found this to be true time and again in all of the years I've been working with electronic gear and computers.