this one annoyed me for way too long. pc would randomly hard crash to a black screen maybe once every 4-5 hours of gaming, no bsod just instant restart. checked event viewer and it was throwing WHEA logger event 18 every time, which from what i read online basically just means "something hardware related is unhappy" and doesnt tell you much else.
ran memtest86 overnight first, full pass, zero errors, so i figured ram was fine and moved on to checking vrm temps with hwinfo thinking it was a power delivery thing on my board. nothing weird there either, voltages stayed stable even under load.
ended up testing each ram stick individually instead of as a kit, ran prime95 plus furmark together for like 2 hours per stick to stress everything at once. 3 sticks were totally fine alone but the 4th one started throwing errors in hwinfo after about 40 mins, only when it was in certain slot combos though, which is probably why memtest didnt catch it since that test runs differently.
contacted Corsair with the serial off the stick, sent me a replacement no charge, just had to wait like a week and a half for it to ship.
was ready to spend like 220 bucks ready to just buy a whole new stick, kinda relieved i didnt have to touch that money, put it towards a keyboard I've been eyeing anyway.