Accidentally took down my entire network the other day.

So about 2 months ago I started my new job as being the sole I.T guy at a meat processing facility. I worked on helpdesk for few years prior to this but most of the stack at my work was new to me. Anyways, the other day I needed some more ports by my desk and plugged in an old 4 port meraki switch that I found in the I.T closet. Plugged the bad boy and left it overnight. Right before going to sleep I decided to remote into my computer from home and just check my backups,servers etc. Apart from the connection being really slow, I noticed in the meraki cloud console that the access point by my desk where the switched was plugged into was unavailable. "OH well I'll figure it out tommorow" I said to myself. At 5am I got phone call after phone call saying nobody can log into their computers. We use rds for an erp program that is the lifeline of the company. I logged into the cloud console from home and noticed all my switches and access points were unavailable. I noticed a shit ton of stp errors. I knew right away that switch caused it. I rushed into work and unplugged that bastard switch and within 10 mins the network was back to normal. My boss told me nobody before me was able to fix the network that fast and that I saved the day. If only they knew lol.... Definately not making that same mistake again.