Here's a basic: Know your equipment ... obviously, right? Well, sometimes the most obvious things are the easiest to overlook.
1st case in point: I went packing with my brother a couple of years ago, he brought his new tent but neglected to run even one setup drill before hitting the trail. He spent 4X as long as he should have to get it pitched, (along with slinging a barrel load of profanities).
2nd case in point: When my CCW class started, two of the students had yet to fire a single round through the sidearms that they had brought to qualify with, they hadn't even read the operators manual! So the class had to wait while they were given instruction on how their weapons functioned; and they never did seem very comfortable with them.
Bottom line: Being familiar with one's equipment is essential to be able to operate it properly and efficiently. Without practice the potential for frustration, failure and injury is greatly increased.