The COG test is very frustrating to fail. It is very similar to baseline TBI tests (for those that have done military stuff) and IQ tests. The important thing that a lot of people mess up is following directions. Sometimes the directions will say "click on the letter that is not the same." The key word missed is NOT and people will end up clicking on the wrong stuff and they'll think they did really well and got all of them, as there is no indication while your testing, but you end up getting 0/25 when you thought you nailed all of them.

AlaskaBound is right though. The biggest reason for people misreading stuff is a lack of sleep, food, hydration and other basic needs that can lead to a bad day. It's a slight representative way of testing to see how people can take care of themselves and follow instructions (sometimes complex instructions) in a stressful environment.

Take care of yourself mentally, physically, relax and read the instructions very carefully and the rest will take care of itself.