A topic with complaints about 4870s biting the dust can be read at XtremeSystems Forums. Several guys report their card died within 48 hours, some of the problems were caused by a corrupt BIOS flash but in other cases may not be related to overclocking, overvolting or flashing the BIOS as one user points out he was running at stock and another unfortunate user reports he has four dead Radeon HD 4870 cards:
Got 4 dead here, as well.This happened to Radeon HD 4870 cards from various brands and because there haven't been any reports of dead Radeon HD 4850 cards it seems safe to assume this issue isn't related to the RV770 GPUs.
To fuel the fire and speculation, both sets we have died in Crossfire mode. (may be a trend from the looks of it)
We're not sure what makes the 4870 cards die as there's insufficient data but at this point the GDDR5 memory is the main suspect although some users report it happened shortly after they put Radeon HD 4870 cards in CrossFire. If you have a dead Radeon HD 4870 please report about it in our comment section and tell us how it happened.
UPDATE: It turns out that most (if not all?) of the Radeon HD 4870 deaths can be attributed to a failed BIOS flash. The reason why this wasn't clear at first is because the problem only arises after a cold boot. If that's the case you can get a fix for your card at Techpowerup.