This is apparently well known elsewhere, but I thought that I might mention it.
I've had an Atmel ICE programmer for a while.
I thought that I might pick up a few spare 6" 10 pin 50 mil IDC cables since they are pretty small and get some wear and tear.
The generic cables arrived (correctly made) and didn't work.
It turns out that Atmel doesn't make their cables 1 to 1.
So if you need a replacement cable, get a "special" Atmel one.