Rashi on Numbers 6:9/רש"י על במדבר ו׳:ט׳

1בפתע — This refers to a Nazarite who defiles himself thus by accident.

2פתאם — This refers to one who does so inadvertently (Sifrei Bamidbar 28:1; cf. Onkelos). — But there are some who say that פתע פתאום denotes one thing, viz., an occurrence of suddenness (by a sudden occurrence, suddenly).

3וכי ימות מת עליו AND IF A MAN DIES עליו — i.e., in the tent where he (the נזיר) happens to be.

4ביום טהרתו [THEN HE SHALL CLIP THE HAIR OF HIS HEAD] IN THE DAY OF HIS CLEANSING — i.e., on the day when he is being sprinkled with the מי חטאת, (the water containing the ashes of the Red Heifer; cf. Numbers 19:17). Or perhaps this is not so, but it means, on the eighth day when he is perfectly clean! It, however, continues: “on the seventh day [he shall clip it]”. But if it had written only “on the seventh day [he shall clip it]” I might have thought that he must clip his hair even if one had not sprinkled him for some reason or other! Scripture therefore also states “on the day of his cleansing [he shall clip the hair of his head]” (Sifrei Bamidbar 28:2).