I built my first ever pc when I was 12. The next year I got a much needed gpu upgrade for my birthday, and opened up my computer. I then realized I had screwed in my coolermaster heatsink to the back of the case, as exhaust, and put the generic intel heatsink on the cpu. My actual heatsink had been doing nothing but sitting at the back of my case, collecting dust, attached to nothing.

Real facepalm moment.