Autumn crocus or meadow saffron has a beauty that belies its deadliness. Autumn-flowering geophytes (bulbous plants and ilk) are uncommon compared to their spring-flowering counterparts; among bulbophiles and other gardeners, Colchicum autumnale provides an easily-grown counterpoint to the typical hues of autumn in temperate climates.
However, it is not without its danger. As is often the case with poisonous compounds, dose matters. Colchicine, present in species of Colchicum but primarily extracted from Colchicum autumnale, is an effective treatment for gout in extremely small doses. Too much colchicine (and one would not need to ingest a lot for this) leads to symptoms similar to arsenic poisoning (according to Wikipedia). Unlike arsenic, there is no known cure for poisoning by colchicine. Animals other than humans are also affected.
While easy enough to identify in the autumn by flowers, the spring foliage can sometimes be confused with edible plants such as ramps or ramsons. Therein lies the danger, particularly for foragers. Ramps and autumn crocus do not co-occur in the wild, with ramps being native to eastern North America. Unfortunately, autumn crocus and ramsons both have a broad distribution in Europe where they can inhabit similar environments like damp meadows, open woods, and riverbanks.