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: 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)
Visible crew/equipment: When we see Nancy, Robert, Dale and Brennan on the boat, as the camera pans past Robert and Nancy, you can see the camera reflected in the shiny post of the boats wheel. (00:14:55)
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: 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 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: 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 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)
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.