Yup, I fried a motherboard years ago, by not seating a Robotics modem card properly. The shop that built the computer put in a new motherboard, and told me that it was under warranty. He knew I screwed up, because I told him that I heard a pop and saw smoke, but I had done a lot of business with him, and it cost him nothing to send it back to the manufacturer and get a new one.
Well in some cases I count that as lucky and others unlucky. None of my PCI cards are screwed in at the moment, because my case is extremely annoying to do them up. Suffice to say if I push the monitor cable in a bit too firmly after moving my PC, I'll get no signal and i'll have to pull on it to raise the graphics card back into the slot! :O I'll sort it soon, it's just one of those 'get round to things' I wasn't so lucky when I saw smoke though, it cost me £90 and two of my components, which I elected not to replace.