Catches or tears in fabric, or obstacles and difficulties that suddenly appear and cause problems.
From Old Norse 'snagi' (a peg or protrusion). Originally meant a jagged point or branch, extended metaphorically to problems that 'catch' you unexpectedly.
In ecology, a 'snag' has a completely different meaning—dead standing trees are intentionally left in forests because they're crucial homes for woodpeckers, beetles, and other creatures that engineer entire ecosystems.
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