always read through the test first. I used to have a teacher who would randomly give tests where you were supposed to do weird things because he would a hide a question that says only sign your name or answer odd questions or even questions or every third questions, etc.

sounds like you got too much inside your own head and worried too much, try to relax a bit next time and give it a look before starting. make sure you have everything you need, and this means the knowledge of what exactly is in front of you and needs to be done. and as mentioned by others, make sure to read it all first to catch any "gotcha questions". also knowing how much to do means you can know roughly how long to give each question etc...

This is a nightmare scenario! Always read the test through first before starting.