I have a cabinet full of assorted screws, nails, nuts and bolts and bits and pieces that I've saved in the course of DIY over the decades. Since I moved to a modern flat, it's hardly been looked at in its dark cupboard corner.
Recently, I noticed that the endless bumps and potholes must have shaken out the bolt holding in place one of my rear panier's supports. This hasn't had an immediate effect on my ability to get the weekend shopping home on the bike, but it can't be doing any good to have all the load carried on one side. So it was time to try to find a screw of the right thread, thickness and (crucially) length to fit the slot. And sure enough, after much delving and sorting, I found it: I have no idea what it was saved from, but it was a perfect fit.
Thirty years of hoarding to save a pound. My Dad would be proud of me.