
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.

unwanted
Not desired; not wanted by someone or rejected.
Er fühlte sich wie ein ungewollter Gast auf der Party.
He felt like an unwanted guest at the party.
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.
unwanted
Unerwünscht and ungewollt can both describe something that someone does not want to have. Unerwünscht emphasizes rejection or the fact that something is not welcome, regardless of intention; ungewollt can additionally mean that something happened unintentionally. For a message that is not welcome, unerwünscht is appropriate; for something sent unintentionally, ungewollt is more appropriate.

Die zusätzliche Werbe-E-Mail wurde durch einen Fehler ungewollt verschickt.
The additional marketing email was sent unintentionally because of an error.
Die zusätzliche Werbe-E-Mail war bei den Kunden unerwünscht.
The additional marketing email was unwanted by the customers.