Not sure if this counts as a "rabbit hole" as meant by the OP, but my rabbit hole has been... bicycle lights. Especially headlights - not only do I gotta have not one, but two headlights for (and on) each of my four active/semi-active bikes, but I've always compulsively bought a pair of new headlights (mostly Bontrager Ion 120s) with every new bike I bought. I even recall at least two instances where I went to the Trek/Bontrager dealer to pick up a new pair of headlights first and then pick up my new Dahon from my favorite sporting store. As you can imagine, I've got many unused headlights (and at least a couple of tail lights) just lying around with no bikes to put them on.