A few things are:
-incorrect prioritizing
-not reading the rules
-not thoroughly testing your robot
-small build time screwup
This list is coming from what happened with our team this year.
When we planned our robot, we didn't prioritize picking up game pieces off the ground at all, resulting in us getting very few (no) points on the board. In addition, we didn't read the rules all the way and had to re-design our robot three times during build. Lastly, since we didn't test our robot during the season (practices were at quarter speed) we missed key driving issues such as our drive train overheating and binding. (After competition we found out that someone forgot to grease our gearbox which caused a lot of the issue, hence, small build time screw-ups
).