If you overclock your graphics cards, watch not to push too far. If you do, all will seem well, but it isn't, the error rate will skyrocket and unless your standing there and watching it you won't notice. You can check the POAM@Home task list and look at the Error link. If you have a lot in the "Error while computing" category something is wrong with the graphics card. Not enough volts, too high core and/or memory. In my case the core was too high. I was losing as high as 19%/day production to errors. I dropped it down a little and all is fine. My 280x's are hitting very close to 500k/day now @1100/1650 1.275v
If you don't overclock and you have a large number of "Error while computing" in there then something is definitely wrong with the card. For reference, I have two cards that have run almost 2 weeks and never gotten a single error so its not something that is supposed to happen.