Earlier this year, when my last in-person student was building his workbench, we noticed some scratches had appeared out of nowhere on every single one of the legs for both of our benches.
I stopped my student, pointed out the scratches, and we went to work tracking down the cause. We were each using our own tools, so I started the process by looking at the tools we shared. The distance and location of the scratches made me think that it was being caused by a burr on the bed of my chop saw. We inspected the saw and found no burr.
That left me scratching my head. Until I realized the scratches were only at the top of the leg. Then I remembered we had been sharing my 12" Starrett Combination Square as we had been truing up that end. I grabbed the square and, sure enough, there was a burr at the bottom corner of the stock. It had probably been dropped or knocked a little too hard against another tool.
I keep a set of diamond needle files (the fine ones with the red handles) on the window sill behind the front bench. So I pulled one out and showed my student how remove the burr.
Then I put on my tweed professor's jacket (you know, the one with the leather patches on the elbows), lit my best pipe and began the lecture. I explained that it's always a good idea to find the cause of a problem like this as soon as you notice it. If you let it go, you're likely to cause more damage that you'll only have to fix later on. It was a good piece of instruction.
As I returned to drilling the draw-bore holes in my set of bench legs, I couldn't help being proud of myself. Surely this lesson would save my student hours of work over the years.
Did I mention that the drill press I was using had a broken quill return spring. It had been broken for months. Oh, I had the part. A good friend bought it for me as a gift. It was buried in some dark corner of the shop.
After drilling the very next hole, the lever slipped out of my hand and the bit dug into the side of the leg. I swore as I added up the amount of time I would have to spend replacing the entire thing.
My student, who knew the drill press had been broken for some time, leaned against his bench, put a comically smug look on his face and began, "You know, as soon as you notice a problem..."
Then I cursed at him, we both had a good laugh, and I decided I had better start searching for that part.