They summoned a spirit. As a gag. At a party.
From the chip strewn, beer misted floor of the living room rose first a hand, clenched in defiance, and then, slowly, a shifting, vaporous shade of a man whose shorn head throbbed with veins of unearthly light. He wore an abused leather jacket and a wispy t-shirt advertising an entity called the Scum Truckers, whose logo appeared to be a demonic skull pouring rivers of toxic waste into his eyes and mouth holes and all that sat below.
They called him up, but could not put him down, this visitor from the Beyond. First, he found a flaw in the magic circle that was meant to contain him: potato chips were only half salt and potatoes lost all magical significance when fried. So he stepped through, struggled for a moment to turn spirit into flesh, but succeeded, and set about drinking the dregs of a thousand beers, smoking strange drugs with weird people, and demanding loud thrash metal to be blasting at all times. He claimed he knew a ritual dance and set about smashing into every one and thing within smashing distance.
Even fleshy spirits must eventually become drunk and asleep on the floor. The party ended and the erstwhile conjurers decided to simply burn down the house and hope for the best.