Bamidbar 35:22

בפתע means BY ACCIDENT. In the Targum it is translated by בתכיף (in close proximity), meaning that he was very close to him and so had no time to be careful about him (i.e. to be on his guard not to kill him; see Rashi on Makkot 7b).

וְאִם־בְּפֶ֥תַע בְּלֹא־אֵיבָ֖ה הֲדָפ֑וֹ אוֹ־הִשְׁלִ֥יךְ עָלָ֛יו כׇּל־כְּלִ֖י בְּלֹ֥א צְדִיָּֽה׃

But if he thrust him suddenly without enmity, or cast upon him anything without lying in wait,