Here's what happens...

Derailleur hanger starts out vertical. Derailleur set screws are set so that the upper jockey pulley stops under the smallest cog in the cassette at one extreme, under the large cog at the other extreme.

Man with bike hits tree.

Derailleur hanger bent inward.

Derailleur now stops under the second smallest cog at one extreme and beyond the largest cog at the other extreme (which is now into the spokes somewhere).

Wheel is rotating, derailleur dives into spokes, rips off derailleur hanger. Rinse, wipe hands, repeat.

Repair or replace the derailleur hanger when you first notice it ain't vertical anymore and before it costs you a derailleur, a wheel and a hanger.