When Quentin Tarantino makes a movie, you know you're going to get a few things as part of the deal. One is an all-star cast. Two is a lot of very violent fight scenes. Third, you're going to get quality. Every Tarantino picture is a big deal, and it's usually in the running for major awards when ceremony season rolls around. What you don't expect is a mistake that leaves you scratching your head.
Jamie Foxx spends a lot of the movie looking like the coolest man on Earth, and his sunglasses are a major part of that factor. The only problem is that back then, nobody wore sunglasses at all unless they'd been prescribed them by a doctor. Even if he did have prescription sunglasses, the style he's wearing weren't around until the 1970s. This is a classic case of style over substance.