Continuity mistake: When Nancy and Robert are yelling at Brennan and Dale for fighting in the front garden the food (green beans) on the plate and the can of Pepsi in front of Dale keep on moving between shots. (00:19:50)
Continuity mistake: After Nancy and Robert ban TV for a week, Brennan throws the ice pack he was holding on the ground. The camera then cuts to Robert, and when it cuts back to Brennan again just a few seconds later, he has the ice pack again and does the exact same action as before. (00:20:40)
Continuity mistake: When Alice writes that she wants Dale to take her now, she places her wedding ring on her plate. However when she and Dale burst into the room the second time, she is wearing her wedding ring again.
Continuity mistake: In the scene where Brennan is showing Dale his sword with Randy Jackson's name on it, his hands switch from having one hand on the hilt to both hands and back when the camera angle cuts to behind Brennan, and back again.
Continuity mistake: In the step brother's bedroom, while Brennan is singing "Let's give 'em something to talk about" there is a Samurai sword on the floor in one shot. When they cut back to the same shot the sword is gone.
Continuity mistake: When Dale and Brennan are in the bedroom after their parents just told them that they are selling the house, Dale is sitting on the bed with the mask on. In some of shots, his legs go from being crossed to different positions and sometimes they are flat on the bed. Also, as Brennan sings and Dale is on the side of his bed, his legs change positions.
Continuity mistake: In the first scene of Dale and Brendan climbing on to the gilded lady when it is in the tree, when they're climbing up the ladder and shout "pirate hats!" there's a man visible in the background, but a couple of shots earlier when they walked out of the house there's an overhead shot showing no-one nearby.
Continuity mistake: When Dale and Brennan first meet at the mixer, a lady with a big blue hat keeps appearing and then disappearing in the background.
Continuity mistake: When the school bullies walk over to Dale and Brennan, the field goes from being bathed in sun to completely shaded over. (00:42:35)
Continuity mistake: When Dale and Brennan are throwing the Christmas presents on their parents bed, the presents on the bed change between the first few shots. (01:00:10)
Continuity mistake: When Brennan is singing at the Catalina wine mixer, and Dale is on the drum set, you notice that the drum beat is not the same sound. It is offbeat. There are more beats than what Dale is producing.
Continuity mistake: When the Billy Joel cover band is performing at the Catalina Wine Mixer, the guitarist is using a Divided by 13 head/cab. This is the green amp located by the drum riser. However, when Dale and Brennan take the stage, the amp has changed to a green Mesa/Boogie Stilleto Ace combo. You can see the Divided by 13 in the shot after the musicians have left, when Robert is explaining how he wanted to be a dinosaur behind Brennan's back, which shows that it wasn't moved by the musicians. (01:23:10 - 01:28:40)
Continuity mistake: When Brennan and Dale first meet, they are standing on the front lawn and they tell each other that they have to be called a certain nickname. When the camera is zoomed back, it shows them really far apart from each other. When it shows Brennan's or Dale's point of view, they are closer together. (00:06:55)
Answer: I doubt it's a person. Jack likely refers to Jack Daniel's, and a handle is usually referred to as a half gallon. Hence the name Jack Handle.