I did The Classic noobie mistake a few weeks ago.
I had my new bike up on the work stand. Wanted to change the stem. Anyone with experience knows you don't do that on a work stand.
I took off the stem cap and the stem and all the head tube innards and spacer rings and washers from the top dropped out all over the floor. I didn't find one spacer ring and had to buy another one.
Lesson learned.