The weirdest macbook pro I've ever encountered..

Today I received a Macbook pro that had a failing hard drive. It was 2 months after the warranty had expired, so customer brought it to me instead.

I went ahead and replaced the hard drive and cloned his old hard drive which had 10.8 installed. (he had his data backed up anyway but I thought it would be nice for him to just have the clone of his hard drive)

That was done and computer was extremely slow.

I figured ok maybe it was not such a good idea to clone a failing HD
So I told him that since it's slow we'll just do a fresh reinstall of the OS Mountain lion 10.8

I tried to install the OS but failed so many times. I was getting extremely frustrated. I kept getting the message
"Can't download additional components needed to install OS X"

No matter what I did, I got that message every time. I removed one ram stick and I still kept getting the message.
Eventually it occurred to me to remove the other ram stick and aha! it finally installed.

After the computer booted into OSX everything looked good except the fact that I had an empty ram slot so I only had 4GB

I put the other 4GB of ram back in and the computer booted and worked fine, however, no programs would install which I find extremely puzzling.
Computer works fine no problems but no programs will install UNLESS I remove the "defective" ram.

Anyone have any input on this? Is this an OSX issue or just the ram?

I've seen weird things happen when a module is defective or mismatched. Check both modules carefully. Are they the same type and speed? We find that occasionally a forum member will assume he can buy faster memory and install it in a machine that uses slower memory. It usually causes problems.

This machine is made to handle 1333 MHz ram. It's the 2011 2.3ghz i7 quad core.
Both ram modules are identical so they must have been running on dual mode prior to this issue happening. It's just so strange that one stick would go bad yet it still somewhat works.
However diagnosis would not run with the faulty ram. Very very weird.

Actually that's not unusual. I've had brand new modules that were DOA (dead on arrival) from the seller. Also, I've had them go bad over time for whatever reason. Who knows? Anyway, the one bad module you have can certainly cause weird problems.