“ On a location with numerous webs, and at least one big spider, there is a something inside a cocoon. It is humanoid in shape, still moving. If the heroes free it (not before they kill or drive away the spiders), they meet a ... zombie!
The poor zombie wandered the dungeon alone, and tried to kill a big living creature (= a spider). The spider used the usual treatment, even if this victim did not look tasty. The zombie can be easily killed as any other zombie. It got but several doses of spider-poison, so can be something worth if it is extracted. You can mention to a druid or ranger the fact the spider had no poison anymore.”
“ A group of Mages sought to create an Elixir of Fertility, to deal with the Curse of the Empty Cradle. After acquiring such things as the 'ahem' vigor of a war deity, and the 'ahem' blessing of a forest goddess, the mages almost created the cure. Instead, the vessel of Divine Virility was spilled, and instead of curing sterility, infused the ancient hill with life. The temple and alter rose, becoming a demonic force of life essence and the fecundity of nature.”
“ Magic is like alcohol, the more that is used, the more it causes a hangover later on and the less judgement one has when using it. If one waits a while after casting a spell, things 'detoxify.' A cantrip or two is like a sip of weak beer, whilst a large creation spell is like a bottle of vodka. Cast something too big and you can die from magic intoxication.”