Continuity mistake: Near the start when Nancy and Robert first meet and are undressing themselves, Robert removes his jacket in a wideshot and his shirt and tie are still on. But in the next closeup the tie has vanished and he is suddenly unbuttoning his shirt. (00:02:10)
Continuity mistake: After arriving to the new house, Brennan is seen in the car as Nancy and Robert talk to the neighbor (Don). Brennan's arm is seen up holding the handle on the interior of the car near the window then down between each shot. (00:06:05)
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)
Continuity mistake: When Brennan and Dale first meet on the front lawn, they stand and stare at each other for a while. With every shot change, it continues to change from either being sunny or shadowy in the spot where Brennan stands. (00:07:00)
Continuity mistake: In the scene of the step brothers' first dinner together, Will Ferrell can be seen picking up a French fry, but in the next shot he is holding a half eaten chicken nugget. (00:08:30)
Continuity mistake: The bottle of ketchup in the dinner table scene, early in the movie, alternates from right side up to upside down, from shot to shot. (00:08:45)
Continuity mistake: After the first dinner scene with the family, Dale finally decides to leave and head upstairs. He places his napkin down next to his plate. In the next shot it has vanished. It then reappears a few shots later. (00:10:15)
Continuity mistake: When Dale is questioning Brennan as to why he is sweaty, and again when he comes downstairs after examining his drum kit, the remote control Brennan has swaps which hand it is in a few times and his hands keep switching their position too. (00:16:45)
Continuity mistake: When Dale and Brennan are fighting over the drum kit, Brennan throws Dale into the gong and it falls over. In the next shot it is standing again. (00:18:15)
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 Nancy and Robert find the kitchen in a mess, after Brennan and Dale's sleepwalking episode, the mess on the floor and counter differs between shots. (00:22:00)
Continuity mistake: In the dinner scene where Derek is over, he is telling a story to Robert about fishing. During the whole scene if you watch Alice who is sitting beside Derek, her wine glass repeatedly jumps from her hands to back on the table instantly. (00:24:50)
Continuity mistake: When Brennan and Dale are in the tree house and Derek comes up to talk to them, the soda can over Derek's shoulder moves around between shots. (00:27:15)
Continuity mistake: When Brennan and Dale are in the tree house and Brennan's brother comes up, Brennan has a magazine in his hand, and each time the shot changes, he doesn't. It alternates between the two. (00:27:55)
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: In the christmas scene where they are decorating the tree, we see Robert eating a christmas-tree shaped biscuit. When he gets up, the biscuit is suddenly larger and more whole than before, where it was almost finished. (00:57:45)
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: At the Catalina Wine Mixer, Brennan Huff's voice breaks the large glass/ice helicopter sculpture. A few moments later, his step father is clapping and the helicopter sculpture is visible behind him. (01:22:45)
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)
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.