I was hired as a firewall admin at an online trading company, then quickly discovered the director of IT was insane, but kept management happy because he made his numbers by keeping his team constantly understaffed; I was told to work on not just servers, but installing Sun servers in racks, running cable, and fixing just about anything plugged into the network.
I made the mistake of showing competence in networking, so was asked to "expand my role" (new title, same salary), and start working on the switches themselves, including executing an "upgrade" to stacked HP ProCurve switches with VLANs (replacing a hodge-podge of random manufacturer switches). The actual upgrade went fine, basic testing (ping) showed everything stable, but as soon as trading opened the next day, everything went to hell, performance dropped through the floor and customers started calling in about trades timing out. Long story short, turned out that Solaris HME cards were unable to negotiate properly with ProCurve switches, half the machines were dropping packets due to duplex mismatches. There's a reason people call the Sun interface cards "Happy Meal Ethernet"
Cost the company approximately $180,000 in direct and customer exodus losses, and was likely a factor in their eventual collapse. I wasn't fired, but management never trusted me again so I saw the writing on the wall, and quit to do consulting work at a (also doomed) dot-com online supermarket.
On the upside, I was able to make thousands in consulting income from installing those same "lock speed to 100 and duplex to full" Solaris scripts on servers for various customers who also had performance issues plugging in Sun servers to cheap switches.