If someone spits on already-slippery floor tiles near a stairwell with sharp edges (or a low-ish handrail next to a long drop), and then someone else hurries over by the stairs with smooth-soled shoes on… oh noooo! Or, if someone spits on the floor who happens to have an infectious airborn disease, or a not-normally-deadly infectious airborn disease which infects someone with a compromised immune system.