Plot hole: At the wine mixer, where Dale is working as a chef , there is a band playing on stage. The drum kit used is the exact same drum kit (chimes and all) as Dale's, as seen in the house. If a band was hired to play they would have certainly brought their own equipment, and there is no way Dale would have brought his personal kit to the event, for somebody else to play. Dale ends up playing this drum kit anyway, when the band walks off and Brennan sings.
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)
Revealing mistake: Brennan buries his brother, Dale. When Dale emerges from the ground you can see a blue material burst out as he emerges. (01:06:10)
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.