(x*y)%10=x%10 if (x%10)%2=0 and (y%10)=6

In words: numbers multiplied by other numbers starting with 6 have the same least-significant digit as before they were multiplied, but only if that digit is a multiple of 2. This is probably due to some rule, but I thought it was interesting as I multiplied a short list by 16 to see I only had to modify all but one of the digits.