ungewollt
unintended
Not intended; happening without intention or a plan.
Das war eine ungewollte Nebenwirkung des Medikaments.
That was an unintended side effect of the medication.

unintended / unwanted
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unintended
Not intended; happening without intention or a plan.
Das war eine ungewollte Nebenwirkung des Medikaments.
That was an unintended side effect of the medication.

The word consists of the prefix 'un-', which expresses negation, and 'gewollt', the past participle of 'wollen' (to want). So it literally means 'not wanted' or 'not intended'.
Das war ein ungewollter Fehler im System.
That was an unintended error in the system.
The exact meaning depends on the context. If 'ungewollt' refers to an action or result, it means 'unintended'. If it refers to a person or thing that is being rejected, it means 'unwanted'.
Durch einen ungewollten Klick hat er die Datei gelöscht.
With an unintended click, he deleted the file.
'Ungewollt' can also be used as an adverb to describe a verb. In this case, it almost always means 'unintentionally' or 'accidentally'.
Er hat sie mit seiner Bemerkung ungewollt gekränkt.
He unintentionally hurt her with his comment.
random / by chance
'zufällig' and 'ungewollt' can both express that something was not planned. The difference lies in the focus: 'zufällig' emphasizes chance, meaning that something simply happened that way. 'ungewollt' emphasizes that someone did not want or intend it. Learners may confuse the words because both can happen without a plan. If you want to say that something happened through luck or chance, 'zufällig' fits. If you want to say that something happened against someone's wishes or without intention, 'ungewollt' fits better.

Beim Aufräumen habe ich ungewollt ein wichtiges Papier weggeworfen.
While tidying up, I accidentally threw away an important paper.
Wir haben uns nach dem Kurs zufällig an der Bushaltestelle getroffen.
We ran into each other by chance at the bus stop after the course.