I had a $cow-orker who once did the exact same thing -- configured an interface to use a /8 subnet mask -- except it was on the new Cisco ASA firewall we were d

I had a $cow-orker who once did the exact same thing -- configured an interface to use a /8 subnet mask -- except it was on the new Cisco ASA firewall we were deploying for one of our customers (a government water utility) and it wasn't a stupid default; he explicitly configured the wrong subnet mask and told the ASA that there were ~16.7 million other devices directly connected to that particular interface! Not surprisingly, they experienced various problems over the next few weeks due to "the firewall" -- including breaking the application they used to provide their vendor remote access for monitoring and maintenance! That had the biggest impact but there were also various web sites they couldn't access, they lost the ability to send and receive e-mails from some other organizations they worked closely with, and so on. Oh, and either Pandora or Spotify (I don't remember which) also stopped working -- she could deal with some of the other stuff but the manager of the organization was NOT happy about that one! Eventually, a few weeks after the firewall replacement, I was asked to go investigate the issues they had been experiencing since the new firewall had been installed, quickly noticed the incorrect configuration on the interface, and became the hero to everyone (well, except for my $boss, who had to reimburse them for ~27 hours (IIRC) of on-site time, but that's another story). Subnet masks matter!