If you want to play a trick on a colleague who is writing a LaTeX document: Open the file, copy the entire contents of the file and paste it at the very end. LaTeX will happily process the file and render the first part. But your colleague will likely try to edit the end of the file and soon feel haunted because his changes to the file have no effect – no matter how often he deletes the resulting PDF file or any intermediate file, how often he renames the original file or moves it somewhere else...
(Just happened to me, but not due to a mean colleague, but due to some accidential vim commands.)
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