rm -r
In UNIX command prompt-speak, it simply means "remove recursively". More verbosely, it means "nuke everything on this directory and every directory under it."
The rm command only removes files, not directories. To remove directories, you can use the rmdir command. But I was raised on graphical user interfaces (Windows Explorer, anyone?), and I'm conditioned to expect widespread damage from a click of a delete button. It gets tiresome typing rm and rmdir separately, especially with lots of files and directories.
rm -r is the lazy way out — it removes files and directories. It's fast, convenient, and like any other delete function, deadly if you're not careful.
My near-death experience happened this afternoon when I typed rm -r [path redacted] to remove files I mistakenly checked out of source control. The instant I pressed Enter, I realized I was at the wrong directory level, and I was in fact deleting a folder containing source for five of my sites — and the Smarty templates for all of my sites — from my production server. That is, from my publicly accessible sites!
I could have just checked it all out of source control again, but some portions of the sites weren't put under source control for very specific reasons. Thankfully, I mirror everything in a test environment so that I can iron out bugs before moving changes to the production environment. So I just copied the test files over to production and was up and running again.
Aside from a few oddities, which were cured by clearing out the Smarty cache, I was maybe borked for about 20 minutes.
Nonetheless, rm -r is one of tho most dangerous keystrokes someone in my field can type. As vigilant as I try to be, sometimes quick fingers prevail.
In other news, I have managed to move the Eponymous 4 official site — and it's "audition" sister site — over to CodeIgniter. They don't look or act any different, which is the point. The sites should act and feel like nothing has changed, even though a lot have. That's how I measure a successful launch.