unfreiwillig
involuntary
Happening against one's own will or without one's own intention.
Er legte eine unfreiwillige Pause ein, weil der Zug Verspätung hatte.
He took an involuntary break because the train was delayed.

involuntary / unintentional
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involuntary
Happening against one's own will or without one's own intention.
Er legte eine unfreiwillige Pause ein, weil der Zug Verspätung hatte.
He took an involuntary break because the train was delayed.

The word is structured simply: it is the negation of 'freiwillig' (voluntary). The prefix 'un-' reverses the meaning of 'aus eigenem Willen' (of one's own free will).
Er musste eine unfreiwillige Überstunde machen.
He had to work an involuntary hour of overtime.
'Unfreiwillig' (involuntary/unintentional) can be used as an adjective before a noun or as an adverb that describes the manner of an action.
Der Streik führte zu einer unfreiwilligen Arbeitspause. (Adjektiv)
The strike led to an involuntary work break. (Adjective)
In the meaning of 'unintentionally funny', the word is very often used in the fixed expression 'unfreiwillige Komik' (unintentional comedy). This combination describes a situation that is funny without that being intended.
Der Politiker sorgte mit seinem Fehler für unfreiwillige Komik.
The politician's mistake created unintentional comedy.
random / by chance
'zufällig' and 'unfreiwillig' can both mean that something does not happen intentionally. 'zufällig' means that something happens by chance and was not planned. It can be neutral or even positive. 'unfreiwillig' emphasizes that someone did not want something or did not do it on purpose; the situation is often unpleasant, embarrassing, or funny. Learners may confuse the words because both have to do with a lack of intention. The difference is: 'zufällig' is about chance, while 'unfreiwillig' is about the person's will.

Beim Vortrag wurde Max unfreiwillig komisch, weil sein Mikrofon plötzlich ausfiel.
During the presentation, Max became unintentionally funny because his microphone suddenly cut out.
Nach der Arbeit trifft Lisa zufällig ihre alte Schulfreundin im Supermarkt.
After work, Lisa runs into her old school friend by chance at the supermarket.