When Nicole encounters the killer for the first time, he puts her T-shirt over her head, but some discrepancy in continuity is evident in the following shots.
Color instead of blood at the start of the movie is obvious, since it's a more watery substance than blood really is and it's applied to the skin, rather than shed with the weapon.
After the killer leaves Nicole's dressing room, he phones back within 10 seconds, impossible in 1971 France, without cellphones.