My point is, how do you know? If it worked fine for regular or fast ethernet, that doesn't prove anything about half of the cable, and if it once worked with gigabit, you wouldn't be asking the question. Unless you tested with a tester that checked all four pairs, there's no way to know if it was a good cable or not.

Regardless, the spec (802.3ab) says Cat5. Period. Not even Cat5e. As Trevor pointed out, the ends of the cable may have been untwisted too much for gigabit. Or if it was a homemade cable, the data pairs may not have been the twisted pairs.

That said, it's possible that Cat6 cabling might give you more leeway than Cat5. That is, it may be that you can make zero mistakes with Cat5, but you can make a few with Cat6. I don't know. That's just conjecture.