What’s the network issue you spent hours troubleshooting that had a ridiculously simple cause?

We have all had one of those problems where everything points to something complicate.

You start checking routing, firewall rules, VLANs, DNS, packet captures, vendor bugs… and hours later it turns out to be a bad cable, duplicate IP, typo, or something equally stupid.

What’s the longest you’ve spent troubleshooting a network problem only to find out the actual fix took 30 seconds?