Not sure on the exact chance of dong it with magery "arch cure" spell, but I've failed to cure lethal poison so rarely it's not even worth noting that you can fail, normal cure spell i don't think has a chance, greater cure pots I BELIEVE have a chance to fail on lethal, but it's very very low, and there's no timer so you can instantly chug another. At 120 healing I've failed to cure lethal poison once also, it actually shocked me as it's never happened to me before, i didn't know you could fail at 120 until that.