Mine;
At the age of about 15 having grown out of my bike I decided to see if my dads would fit. I decided it would but didn't like his saddle and wanted to transfer the one from my bike.
I didn't realise/think that the saddles might be swappable between seatposts so decided to fit my seatpost to his frame.
So I spent a happy afternoon bashing the top of my seatpost with a 2lb hammer in an effort to make it fit into the 'slightly too small' seat tube on my Dads bike.
Eventually the frame split down from the seatclamp slot.
Being a coward I shoved it to the back of the shed and didn't say a word. It was about 2 years later I was presented with the results and asked to explain.
More recently I managed to refit the lowers the wrong way round when servicing my forks although that was relatively easily resolved.
I got most of my spannering disasters out of my system on my first motorbike, almost wrote it off with an 8mm spanner once.
