Very early in my career I discovered some mission-critical network devices at the hedge fund I worked at that would have caused a network loop if either of the two machines were ever rebooted. The two servers were related to some feed data and I had noticed that they were cabled together oddly based on how their function was described to me.

The estimated cost of downtime for these systems was something like $7 million per minute. I had raised the issue to a couple of the staff responsible for the machines and to the networking team but was completely dismissed because "there is no way we would have hooked them up that way" and because I was the FNG.

I then raised the issue again at the weekly group meeting because it seemed important -- somebody was dispatched to check visually and came back to confirm what I said. It was a big deal -- the networking team had about 2 weeks of emergency work to do to resolve the issue cleanly.

EVERYONE was angry at me. Even though I had just averted a catastrophe for the company, I made everyone look bad by doing it and particularly because of my status/position on the team. It was an important lesson learned.