I had just finished installing Solaris on a workstation and was in the midst of some post-install tweaking with multiple xterms connected locally and to other machines open.
I finished up and wanted to make sure the workstation came up properly so I found an xterm window with a generic root prompt and typed 'reboot'.
Turned out that I had lost track of which windows were connected to what and our production server was merrily heading down to init 6...
I don't think I ever sprinted down a flight of stairs so quickly.....
The only (small) point of pride I have about that incident is that I fessed up immediately and took steps to minimize the collateral damage.