There is one universal mistake that almost every child has committed when playing with legos. This mistake is sadly unavoidable and is never mentioned in the instruction manual.
A child is simply not strong enough to press each piece 100% into each other resulting in really weak end results. These weak creations will self destruct at the slightest of impacts resulting in devastated children. Sadly it is very hard to completely fix a finished product, but it is definitely possible to reinforce one.
Why humans fear legos:
1. Stepping on a certain lego bricks will deal impossibly high critical damage to humans.
2. It is almost impossible to remove flat pieces from each other if they are the same size. I've actually broken a knife whilst trying to separate lego pieces before.
3. One missing piece can ruin a build. You never really know if you've got every single piece you need. The seemingly random assortment of extra pieces makes it seem like it's a bit more random what you end up with.