I've known people to hurry to get the test over with WITHOUT READING the instructions. "Do not answer any questions. Put your pencil down." And 23% of the students completed the entire test, and FAILED.
Wow. I once had a test, in which 27 out of 30 were "B" (from a,b,c,d choices). There were several people who thought that there is something wrong here and started answering something else instead, when they weren't 100% sure what was right. What a prank :D
I've given tests to my students where I've made the correct answer the same letter 5 or 6 times in a row. It's amazing how many students will be so freaked out by this that they start giving the wrong answers. Its how I weed out the weak ones.