Hmm, I've had too many to count.
Let's see-knocked laptop off of a case and the power connector shoved the socket in and chipped the mboard.
Threw my Dell Axim X30 high into the air after the touch screen stopped working, and cracked the screen. I later learned that if I hadn't cracked the screen I could have paid $5 for a replacement touch overlay, instead of $40 for a whole new screen.
Installed Kubuntu 10.10 which lacked the option of not installing the GRUB bootloader as was included in earlier versions, so I ended up overwriting my MBR and then when I deleted the Kubuntu partition, I was unable to boot my system and after many hours of frustration at the command line in a LiveCD I finally just reinstalled Ubuntu 10.10 back onto its partition.
Then I accidentally lost my Wubi installation while uninstalling programs manually
, lost some important files.
Dropped my screwdriver into my machine while it was on, it shorted and shutdown. It survived, though
Darn thing has had a screwdriver fall in it twice now, the first time was when my friend did it and he gave me the machine because he couldn't get it to turn on, which I found was due to the PSU
And I'm with everyone about the Molex connectors, I haven't torn myself open on them thankfully, but I tore several straight off the wires with pliers while trying to get them out of several hard drives.