Panaeolus olivaceus is a semi-rare and widely distributed little brown mushroom which contains the hallucinogen psilocybin, is often mistaken for Panaeolina foenisecii, and is distinguished by its black spore print and darker gill coloration when mature, as well as a slightly thicker stem. It is even more easily mistaken for Panaeolus cinctulus and can be distinguished from that species by its more campanulate cap shape when young and slightly roughened spores. It is also easily confused with Panaeolina castaneifolia, a species which has spores that are dark brown and significantly more roughened.