I know nobody wants to hear this, but I firmly believe that a lot of these strange, unrepeatable problems are due to bad burns on the installation disks.
I see a lot of these problems from relatively new users, and almost none from experienced users. Distros use very aggressive data compression. This means the data density is quit high, making the burn process more difficult for the cd/dvd drive to accomplish without errors. With music or video this isn't a problem. A bad bit once in a while goes completely unnoticed. Not so with program code. Every bit counts. Even if the data is error free on the disk and the md5 sum checks correctly, the signal strength of each and every bit may not be up to par so the read process might not be consistently error free. It is an established fact that even if the md5 sum of the disk checks correctly once, you can still have a disk that won't install correctly every time.
Experienced users are aware of these issues and always burn s-l-o-w and in a RAW burn mode. They have learned over time that this is necessary to get a consistently good burn. They will also use a brand of disk that experience has taught them works good in their burners.
With many new users, they either don't know or don't believe this is important and wind up with something less than a perfect install. Most times this goes unnoticed as Windows users are used to random crashes that come and go with no rational cause. Long time Linux users don't accept this kind of behavior as normal, because it isn't for Linux.
At any rate, that is my theory.