Luckily my biggest screw up didn't happen, but came incredibly close:
I was logged in as root on a production server, branching a live site into a testing site. Now the thing I should point out is that this server for some strange reason defaulted to the root directory on login (as in '/' not the root home folder). Now to get the site working I knew I would have to change the user and group on the files so I typed this command:
chown -R **** psaserv ./*
And hit enter...thankfully I had forgotten the syntax of the command (it's 'user:group' not 'user group') and I realised I hadn't changed directory while fixing that mistake.
I could have lost my job with one little command lol.