So, I bought a Ryobi cordless drill a few years ago. The battery got completely drained since I don't use drills very often, and it won't hold a charge. So I looked at the battery, and the only indication of what it is was "Ryobi 9.6 volt." So I went online to buy a replacement, and there were plenty of "Ryobi 9.6 volt" batteries. There were absolutely no indications other than that for what sorts of batteries fit into which tools. So I bought one.
And of course, the one I got, which says "Ryobi 9.6 volt," has a completely different connector than what's on my charger or drill.
For $5 more I could have just bought a new damn drill. Ugh.
Of course, now I see that there's a "battery number" that I was supposed to match. GOOD THING THIS WAS OBVIOUS.