the.com/pitfall

means A hidden danger or mistake-in-waiting that catches you off guard, especially one easy to overlook until you're already in it.

from Literally a covered pit dug as a trap for animals (and sometimes people) — 'pit' plus 'fall,' as in the thing you fall into. The word has meant an actual concealed trap-hole since medieval times, and only later widened into the figurative sense of a hidden hazard in a plan, argument, or path through life.