The way none of the characters can keep a straight face when reading out the card in the line up was not scripted, but resulted because the cast had had a rather good lunch prior to filming, which included some wine, and were unable to get through what was scripted as a serious scene with straight faces, because of the nature of the scene - supposedly very serious, but they were being asked to utter this rather absurd (and obscene) phrase.

Bryan Singer, the director, played Keyser Soze's feet in the opening sequence.

An interesting point here. Some may have wondered what is the significance of the name "Keyser Soze". As many may have guessed, Keyser means "king". Soze is a word whose English equivalent is the word "verbal". Therefore, the identity of this mystery bad guy is spelled out in plain sight in his name.

The personal bios of the characters are handed out in the order that they die in.

Not really a mistake per se but still. On the cover for the Usual Suspects i.e the box you see the five key characters all lined up as they were in the film. On the box Keaton is set in the centre, but in the film he is not in the centre, he is fourth from the left, next to Kint. Also Hockneys clothes have changed colour. Kints shirt and sleeveless jacket have reversed colour too. You could argue that its just a picture for the box and not meant to be taken directly from the film but if so, then why bother getting everything else right?

Kevin Spacey apparently glued his fingers together in the film to give a realistic paralysed affect.

In the lineup scene, when the officer asks Benicio del Toro to repeat the phrase "in English, please," the voice is actually the director. That line was not scripted. The director actually wanted to retake Benicio's line because he didn't think it was understandable. He ended up keeping it because he thought it was funny.