I RMA'd some DDR3 memory which appeared to be a faulty stick. The RMA has been updated with "Tested - NFF". Here's the details I provided when I logged the RMA:

Mobo: Gigabyte GA-P55A-UD3R (stock timings)
CPU: i5 750
Memory: Corsair 4GB 2x2G CMX4GX3M2A1600C9 XMP
Fault: When both memory sticks installed BIOS only shows 2GB (should be 4GB). Win 7 reported 2GB

I tried installing each stick in the same memory slot one at a time (in slot DDR3_1). The first stick was fine but when I installed the second stick on it's own the PC refused to POST.

I've since bought another set of the same memory and installed it with the same settings, same slots etc and it works fine.

This appears to confirm that 1 stick was faulty even though when tested it passed MEMTest. What's the next step ?