Dumbest thing I did was a bit of a mistake at work.
Cisco switches (and most other managed ones) include something called Spanning Tree Protocol which stops you from accidentally wiring a loop into your network and flooding it with traffic. When you plug something into a port it takes 50 seconds to study the network and work out if it can safely enable the port or not. Because that 50 second delay is annoying sometimes, it's possible to turn STP off. Our main head office switch had it disabled.
Then one day we added a second switch, and so I figured I'd better turn it back on for safety. So at about 10am I went ahead and enabled it. And STP did what it should and disabled every single port for 50 seconds while it checked for loops in the network.
I even had users in New Zealand phoning up to ask where their application went. :whistle: