The swirl designs that are left behind in the toilet after a hefty poop.

when you're hungover as shit after a spending a night drunk as a skunk

Running around a campfire or bonfire while farting the whole time, thus producing a circle of crap smell for everyone around the fire to enjoy.

Origin: This weekend camping, crop dusting turned into a crop circle when I dusted everyone around the camp fire which then turned into the crap circle because it smelled like crap

circles on underwear, skivvies or other pantswear that show up and may be the work of aliens (such as Klingons or other off-world groups)

when you have sharted and there are unexplained sacred geometric shapes in your briefs