I'll start with the standard disclaimer: It was many years ago, I had just started, I was a noob, honest!
I was trying to debug something on a Cisco router (don't even remember what it was), but I just couldn't figure out what exactly I needed. If you've ever tried the debug command in IOS, you'll know what I mean. Every command has a ton of subcommands, arguments, options, etc.
Finally, I came up with a brilliant idea! I'll just "debug all" and then strip out the useless stuff. (I can hear the groans from the other people who've tried this).
Long story short: it killed the router. It just happened to be the core router for the ISP I was working for. 30,000 people suddenly had no internet access.
Yes, it was dumb. Yes, the ISP should not have been letting newbies run around on core routers unsupervised. Yes, we should have had hot standby. Etc, etc. :)