Actually this happened in the lab. I know exactly who did this because he told me: we were discussing what had happened and he said "Oh yeah, Daniel and I needed to connect this Windows machine to the intranet quick because we had something urgent to do, and we connected all the ends of the nest of ethernet cables at random until the machine connected. And then we left everything as it was." But bad luck for us, their machine was connected, but so was that fatal cable on both ends. It just happened that their machine kept working well enough for them to finish what they were doing without noticing the problems rightaway.

And in case you wonder, there's no penalty in our company for owning up to honest mistakes, so that's why he readily admitted to it. Only people who never do anything never do anything wrong.

In my experience it's either someone doing it on purpose, or someone accidentally pulling the wrong cable out of a rats nest.

Basic unmanaged switches often don't have any sort of protection, and on some fancier managed switches it's disabled by default (no idea why)

We used to use Malwarebytes Corporate Edition at work.

One afternoon all of our web servers stopped responding to traffic on port 443. I could RDC into the servers, and I could ping them, but most traffic wasn't being passed properly.

Despite not having made any changes, I did everything I could think of to get them to work. I tried moving them to different switches, different static IPs, Wireshark showed packets flowing, but no web traffic.

I left the office. It was around 8 PM and I had been banging my head on my desk trying to figure out what the hell was going on.

I came back around 10 PM, mind clear and stomach topped off. I worked a few more minutes, then heard the Outlook ding.

Mass email from Malwarebytes CEO. Bad update. Blocked all class B IP addresses by mistake (guess which class we used). Mea culpa. So sorry. New update fixes things.

I immediately uninstalled MWB CE and boom. Services restored.

The next week we got our licenses refunded by our VAR and we never used that product again.

"In theory, theory and practice are the same. But in practice..."

Yeah I keep calling them hubs incorrectly...

There is such a thing as a small 1Gbps hub that are designed to just handle a small network. They scare me as they are cheap on Amazon and could theoretically bring a network to its knees if a random user finds a port that isn't authenticated.

IT tech here, yes, yes it can.

Network infrastructure is both increadibly smart while also being dumb in other ways.

To do an ELI5 answer:

Imagine you have a container of pearls that you need to sort, red, green and blue pearls all need to be dropped into a red, green or blue hole.

The container is being refilled, but slow enough that it only gets a new pearl once you have sorted the previous.

The holes are connected to pipes going to separate buckets.

Everything is fine, but then some adds a new hole that is muticolored and tells you that all pearls should go there.

You tell your friends that you have a faster way to deal with the perls and to send you their pearls.

The new hole also has a pipe, but that is connected to the container that recieves pearls, so every time you drop a pearl into the new hole, it appears in the container again.

So now you have a situation where you not only get your normal ammount of pearls, but everyone else's pearls and you also get every pearl you send back again.

You are smart and quickly realize that something is wrong and call for your teacher for help, networking gear don't have that capabillity to understand that it is wrong, it just looks at each pearl and not the big picture.

If we go back to the real world, we have developed tools to deal with this situation, we have protocols line spanning tree which can have switches speak with eachother and figure out if there is a physical loop before sending traffic through it.

There are other tools as well, but they all need to be configured and to be honest, it is easily forgotten or made a low priority since it happens rarely.

It is something that is often implemented after a big outage.

Certain types of broadcast traffic always get re-broadcast from of every port on a switch. So if you directly connect two ports, and you get some broadcast coming into the switch, that broadcast will loop forever across that loopback, and then get propagated repeatedly until it hits a broadcast boundary. It's surprisingly difficult to prevent even with managed switches unless you are willing to hand manage every port and significantly restrict the kind of network services which can flow through it.

Some devices can detect these loops and break them, but that can have other unintended impacts if your network is designed (some would argue poorly) around using dumb switches to multiply limited Ethernet drops at the edge.

Yeah I've had a wireless uplink between two Unifif AP's on the same switch, the only non Unifi switch, come up by itself and caused a loop. Unlikely that switch only had a 1G uplink to the next switch, all the rest were 10G links, so it mostly only affected devices on that switch.

Edit: thought I'd just say that since then, I always disable wireless uplink on all AP's, and the global system setting, unless it's actually used, and only on the APs that need it.