The very first time I took apart an iBook (also to replace the hard drive) and put it back together, I put one of the long hex screws in the middle, instead of the short one.

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OMG. I did that as well!

I was concentrating so much on the screw that I didn't notice it was the wrong one. When I turned the iBook over, I saw the keyboard bulging up in the middle. Luckily for me, it didn't seem to do any harm as the iBook still works fine. But I don't think I'll be going near any more of these any time soon