If multiple versions of a test are given, students must indicate the test version in the top-right corner of their sheet beside the Scantron logo. Stray marks outside of this region can cause the grading process to abort and error out.
HINT: Rather than having students indicate a version themselves, you can always put a version code on each sheet before giving to the students. If you take this route, consider using a highlighter or marker strike as opposed to writing in a letter version code (e.g. red vs. yellow version as opposed to A vs. B versions)