I've got an early 2011 13" i7 MBP, OS X 10.10.3 (the public beta version before the one released last night) with 10 GB of RAM – one of the original 2 GB modules in the bottom slot and an 8 GB module that's now a few months old in the top slot. Yesterday morning, I opened my computer to reveal a black screen after it had just been working a few minutes ago. I tried to turn it off and back on, and ended up getting the three-beep pattern that indicates bad RAM. I opened up my computer's back, and here's what I tried (and what happened when I attempted to power on after trying it):
Pulling out and reinserting the RAM - no change.
Pulling out the 8 GB and trying to boot with just the 2 GB - no change
Pulling out the 2 GB and using just the 8 GB in the top RAM slot - no change
Putting the 8 GB in the bottom slot - it booted fine!
Adding in the 2 GB in the top slot - no problem!
"No trouble found." -Apple Hardware Test (both basic and extended) last night
So, here's my question: How'd my RAM spontaneously go bad enough to cause the beeps of death, and how'd the solution end up lying in switching which module goes in which slot?