Continuity mistake: When the man is laying out the photo files on the desk, he puts down files one, files two, and then file three. In the next closeup the position of photo film number three has changed.
Visible crew/equipment: When Kakihara and Karen are walking up the stairs at the apartment building and the camera is following them, you can make out the shadow of the cameraman on the actors and the walls several times.
Visible crew/equipment: Near the end of the movie after Ichi cuts Kaneko's throat and he is crawling towards his son Takeshi, you can for a brief moment see the tube with which the fake blood is pumped to his throat.
Revealing mistake: When Kakihara is smoking in the beginning, you can tell the cigarette is not really lit, and that the glow of it was added later using computer graphics. The easiest way to notice this is to notice how no smoke is coming off of the cigarette itself.
Revealing mistake: When the one twin rips the man's arm off, the arm he is holding is not an entire arm, but more like 2/3 of an arm. It is ripped off slightly above the elbow. But in the next shot, you see the man is missing his entire arm almost up to the shoulder.
Revealing mistake: When Kakihara is attacked in the streets, protective padding is visible under his clothing. Look for it when he bends over backwards without falling over, before he removes his piercings.
Deliberate mistake: There is no way Ichi's blade could have cut the one man vertically in half, as shown in the movie. The blade is only a couple of inches long, not nearly enough to slice through the man. When we see the blade shooting through the back of him, it is obviously a different, longer blade.
Revealing mistake: After the girl has her nipples sliced off (in the uncut edition) in some of the shots where you see her dead body, you can tell her nipples are obviously still there, just being disguised (poorly) with makeup.
Deliberate mistake: When Suzuki has the boiling oil poured over his body, his skin doesn't react quite realistic. You only see it scar up a little bit, when realistically, the heat would be melting the skin right off.
