I accidentally untarred archive intended to be extracted in root directory, which among others included some files for /etc directory.
I went on to rm -rv ~/etc, but I quickly typed rm -rv /etc instead, and hit enter, while using a root account.
Because the book’s page 1 is the pdf’s page 41, everything before is numbered with roman numerals :)
I also wasn’t expecting anyone to try and read with a browser or reader that doesn’t show the current page number