Typing the wrong command can severely cripple your computer. A wrong switch or wrong option is all it takes. One command that I learned to avoid a long time ago, based on what experienced users told me, is rm -rf /. The rm command is what you use to remove or delete a file or directory.
By itself, rm will delete a file. With the -r or -R option, it will recursively delete a directory. The -f option will force the action. So, you can imagine what rm -rf / could theoretically do to you your system. But will it?