Many people have given great suggestions for the most destroying commands, but most result in an immediately borked system. While inconvenient, that doesn't have a lasting impact on users who have backups.
I propose writing a bash script set up to run daily in cron, which picks a random file in the user's home directory tree and randomizes just a few bytes of data in the file. The script doesn't immediately damage the basic OS functionality, and the data degradation is so slow that by the time the user realizes something fishy is going on a lot of their documents, media, and hopefully a few months worth of backups will have been corrupted.