This is kind of a corollary to Penultimate Horror. After defeating said monster/genetic/evil/horr or/thingy, it falls to its knees and lets out one final shriek of agony before collapsing, blood pouring from its wounds. Confident in its demise, you turn away to leave this godforsaken place. A squishing noise emanates from the thing's corpse. Reasonably startled, you turn to find that it is nowhere near dead: it has started to mutate uncontrollably. Poison gases and acidic blood spurt erratically from its body. Limbs grow, snake outwards, shrivel, are replaced within seconds. This new terror is a hundred times your better in singular battle: your only choice is to run. And run you do.