Maybe it's a lens, maybe it's an accessory or even a piece of software. Maybe it's a piece of junk, or maybe it was so good you immediately upgraded it.
Perhaps it has a bit of regretful stigma due to the price you paid.
I know some people will just sell unused gear so maybe yo don't really hold on to things, but I was wondering what gear people kept around particular but didnt use AS much.
Mine would he my teleconverter 1.4. I bought it for my 70-200f4l, and while it's fine, I found I just didn't need to add 80mm all that much.