Continuity mistake: During the song at the end, there is a zoomed out shot of the entire crowd. You can see Phineas and Candace both screaming with their hands in the air, their eyes closed, and Phineas is not holding a camera. When the camera instantly cuts to a close up of them, both of them have their mouths closed, their hands down, their eyes open, and Phineas is holding a camera.
Continuity mistake: In the scene where Doofensmirtz talks about how the sign blocks his view of other people's pain and misery, his window faces the sign. Then in the next shot where the laser blasts the sign, the Doofensmirtz evil inc. logo is facing the brick sign and the windows are facing the other way.
Continuity mistake: In the episode 'One Good Scare Ought To Do It', when Phineas is carried into the air inside the haunted house, there is an aerial shot showing the Fireside Girls and Isabella, who is wearing a yellow sash like the other scouts as she shouts, "Phineas!" There are two problems here. First, Isabella wasn't wearing the sash before at the haunted house, until this particular shot. Also, between the shots before Phineas falls, her sash disappears, reappears and then disappears again. It reappears after Phineas falls, and Isabella says, "Quick, everyone! Sashes!"
Plot hole: When Linda (Mom) gets home from her cooking class and greets the boys, she sees nothing out of place. However, just a few moments before, we see the giant laser in the corner of the yard that they forgot to attach to the laser-inator. Wouldn't she have seen this?
Continuity mistake: When Perry is tied to the firecracker he uses a laser watch to burn himself free, but in the next shot it's gone.
Continuity mistake: Near the end of the "Love Handel" concert the people in the audience switch from being close together to spread out leaving a path between shots.
Continuity mistake: In the episode "One Good Scare Ought to Do It", Candace arrives at Jeremy's house and is wearing a belt. (It's very clear as she yells "These are designer jeans!") After Susie's attacks, when Jeremy comes out Candace is on the ground with the dog still attached to her rear end and the belt has disappeared. In the next shot of Candace, the belt has returned.