Continuity mistake: When Bob is in his cubicle before Mr. Huff shows, the pencil holder is on the left side of the desk, next to the computer. When Bob jumps up, but before the pencil holder's knocked down, it's in the middle of the desk. Then when he picks it up it's closer to the phone, but still in the middle of the desk. As Mr. Huff turns to leave, it's still there. But when the camera cuts to an overhead shot, the pencil holder's jumped to the opposite side of the phone, perched on the edge of the desk, where it falls to the floor.
Continuity mistake: When Mr. Incredible and Mirage are having dinner next to the wall of fire, the long shot shows them at opposite ends of a very long table. All of the following close up shots of Mirage, shown over Mr. Incredible's shoulder, show them as being much closer together, at a much smaller table.
Deliberate mistake: Right after Mr. Incredible first lands on the island to find the Omnidroid, he starts to stretch and the camera angle changes, showing us his fat stomach that sticks out of his shirt. During the rest of the movie the suit is a one-piece, so this wouldn't be possible.
Continuity mistake: When Helen and Edna go into her lab and sit down at the "viewing" table, E sets the pitcher down on her side of the table. A couple of shots later the pitcher is on Helen's side of the table. Also, when the table moved back really fast, the items on the table would become misplaced, but they remained stationary.
Continuity mistake: In the dinner table scene, when the family "cleans up" in order to answer the door, Dash's hair is all frizzled and standing up from running around and fighting with Violet. When Lucius shows up, Dash spits some water into the air for Lucius to freeze. Dash then says, "Aww, I like it when it shatters." In that one quick cut, Dash's hair is all neat, almost no hairs frizzled. In all subsequent shots of Dash, his hair's all frizzled again.
Other mistake: Near the end of the film when Jack-Jack is manifesting various abilities, his first display is of the human-torch-type of superpower. This would have burned away all of his clothing, yet (for obvious reasons) his diaper remains intact. This is particularly apparent as designer Edna Mode's costume for Jack-Jack was earlier shown as fire-resistant, but he was not dressed in his outfit during this scene.
Continuity mistake: In the scene with Dash and Elastigirl in the principal's office, Dash's shirt keeps changing between shots. First it has a strange "X"-shaped fold on it, then it changes to relatively smooth, then BACK to the "X", and so on.