When Mookie throws the trash can through Sal's window, the reverse shot from inside the restaurant reveals the main camera, in the lower left portion of the screen, dollying in towards the window as it's smashed.
Sal places a piece of pizza in front of Buggin' Out before they have their argument. After the argument, Buggin' Out sits down to eat his pizza. The piece of pizza looks different than the one Sal gave him.
Right before Sal begins to bash the radio, you can see Radio Raheem's arms on the radio as if he's leaning on it. The second Sal starts to bash the radio, you can see that Radio Raheem is standing a few feet behind the counter.
When Mookie delivers a pizza right after the Bugging Out/Sal incident, Smiley is seen just outside the window of the Pizzeria on the left, as Mookie leaves with the pizza, and turns right. When Mookie arrives in the apartment building to deliver the pizza, Smiley meets him walking down the stairs. It is implausible that Smiley could have been in those two places within the time period shown.
The goof item below may give away important plot points.
In the final scene, when Sal throws $500 to Mookie, he throws five bills, and Mookie throws two back at him. When Mookie goes to pick the money up again, he picks up two bills, but two more are visible in the frame, lying on the ground.