Even the most experienced PC builder can easily forget the importance of RTFM. When I built my FX-4350 based server (albeit in a bit of a hurry), I forgot to check which motherboard DIMM slots should be populated in a two-DIMM scenario. My motherboard is a Gigabyte GA-990FXA-UD3 990FX Socket AM3+ – it has four DIMM slots but I only wanted to populate two with 16 Gb in total. Needless to say I picked the wrong ones – and the performance was noticeably slower than expected.
RTFM! Or even easier … look at the colour of the DIMM sockets! Well … if your motherboard has different colours for each bank – only on Rev 4.0 and later did my motherboard have different coloured socket pairs.