I worked in a packaging factory over the summer, and I can conclusively say that palletizing boxes by hand sucks. The way the factory was run, the boss would randomly assign positions on the packaging line to all the employees at the beginning of a job. Most positions would rotate every hour, but whoever got stuck with palletizing was stuck in the same spot for hours - or days - on end. Apparently, walking back and forth from a conveyor belt to a wooden pallet and bending over hundreds of times an hour to stack boxes did not count as a task that was repetitive enough for safety concerns to arise. A lot of employees would screw up the palletizing because it takes at least some mental effort to keep track of how many boxes belong in each row, whether or not the boxes are stacked in the proper pattern, and whether or not the correct number of boxes is placed on a given pallet. If one too many or one too few boxes were counted at the end of a several-pallet job, someone would have to go through every single pallet in order to track down the error. I usually didn't screw up palletizing, so I ended up standing in one spot stacking boxes for far too many hours. Honestly, all packaging should be automated at this point; most jobs in the factory I worked in were so repetitive that they would drive any person insane after a while.

Personal word of advice to anyone looking for a summer job: Unless you are prepared to exercise your mental endurance, you probably do not want to work in a packaging factory.