You pass through a woodland with a floor that seems remarkably clean of debris. Suddenly your attention is drawn to a twisted bit of wood by the trail side, a short broken twig with several stubs of branches along its length and most of the bark missing. When you reach it, the twig blurs and turns into a small brown lizard, caught in the middle of its body-twisting run. The little reptile darts away down the trail with impossible speed, no doubt magically enhanced, and is lost to sight in a moment. Now someone knows you are coming...
What if potions and tonics had negative side effects? Potions of increased strength cause violent tempers, or damage joints. What if potions of dexterity caused tremors, or palsy?
What if they were addictive, and had withdrawal symptoms?
Could someone make a bad batch? No one use the brown strength potion!