Many schools use scantron machines on multiple choice tests to automate the grading process. This method involves having a test booklet and a separate sheet to write your answers, seen here. This separate sheet will be sent through a machine that detects the answers, as long as you use a trusty #2 pencil. The OP was expressing the frustration that comes from getting out of sync between the booklet and the answer sheet, either by skipping a question or by forgetting to fill in an answer. If you ever take one of these tests, always mark the answer booklet as well as the scantron sheet. Makes finding your error in that situation much much easier.