The problems that are frustrating are the areas you know really well but are not getting an answer that matches the 4 options. At some point you have to move on.
For me the biggest problem can be stupid mistakes. I often times will forget a squared term or something like that. Mistakes like this are not always easy to find.
Also when I am calm I write clearly and precisely, but my penmanship deteriorates with stress.
Hopefully I will be okay once I am well rested. I know some of the dumb mistakes are due to being tired.
Units can be a killer too. Sometimes you might feel like rushing and not carrying through the units, but this can often times really burn you. That time unit in gc can be the difference!