After a crash on Jan 7, I began to order stuff to replace stuff that was destroyed or damaged. In addition to that, I ordered a cassette to reinvigorate an old 7 speed wheelset that was recently given to me. It's intended to be a spare wheelset for the bike that I crashed.
I ordered a new chain without checking if the existing one (less than a year old) would suffice. I just assumed that I would need to swap chains when switching wheels because the old wheelset has a 24t large cog and the new cassette had a 28t. It turns out that the optimum chain length goes from 53 to 54 links. Oh well. I'll probably want a spare chain.
I believed I had ordered a 12-28. I was certain of it. It's an 11-28. (11t on some older Shimano cassette bodies have been known to be problematic according to Sheldon Brown)
I should have ordered an HG50 cassette. I ordered an HG30. Oh, well. It fits and $20 delivered is not a significant investment - about right for a wheel of unknown mileage/condition.
The order was made 1/11, but I'm just getting to a point where I have the physical ability to replace a cassette. So, I'm only discovering these errors now.