to happen / circumvent / be bloodshot
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to happen
To happen unnoticed as an error or oversight. This form is always used impersonally with the dative (jemandem unterläuft etwas — something happens to someone by mistake).
Mir ist beim Kopieren der Daten ein Fehler unterlaufen.
An error happened to me while copying the data.
'Unterlaufen' is an inseparable verb. The prefix 'unter-' remains firmly attached to the verb 'laufen' in all tenses and sentence types and is never placed at the end of the sentence.
Dem Team ist ein schwerwiegender Fehler unterlaufen.
A serious error happened to the team.
When an error happens unnoticed, an impersonal construction with the dative is used. The person to whom the mistake happens is in the dative, and the auxiliary verb is 'sein' (to be).
Mir ist bei der Berechnung ein Fehler unterlaufen.
An error happened to me during the calculation.
In the meaning 'etwas umgehen' (to get around something), 'unterlaufen' (to circumvent) requires a direct object in the accusative. The subject is the person acting actively, and the auxiliary verb in the perfect tense is 'haben' (to have).
Die Firma hat gezielt das neue Gesetz unterlaufen.
The company deliberately circumvented the new law.
In the medical meaning, 'unterlaufen' is almost always used as the past participle in combination with 'sein' (to be) and describes a state. It then functions like an adjective.
Ihre Lippe war nach dem Sturz geschwollen und blau unterlaufen.
Her lip was swollen and blue-black after the fall.
to happen / pass
'Passieren' and 'unterlaufen' can both describe that something happens, especially when it was not planned. 'Passieren' is very general and fits many kinds of events: something good, bad, or simply unexpected. 'Unterlaufen' is used mainly for mistakes or oversights. It sounds somewhat more formal and is typically used with the dative: 'jemandem unterläuft ein Fehler' (someone makes a mistake by accident). Learners may confuse the words because you can often say 'Ein Fehler ist passiert' (A mistake happened). But if you want to emphasize that someone made a mistake by accident, 'jemandem ist ein Fehler unterlaufen' is more precise.
Bei der Planung ist ihr ein kleiner Fehler unterlaufen.
She made a small mistake in the planning.
In der Schule passiert heute etwas Ungewöhnliches: Der Unterricht beginnt erst um zehn Uhr.
Something unusual is happening at school today: classes don't start until ten o'clock.