It's actually amazing how much you can get done without a phone distracting you, but it's expensive and rarely used, and paper is fine for occasional initial brainstorming.
Mechanical Keyboard. I tried one with no page up/page down/home/end keys and hated it. I also wasn't a fan of how separate keyboards keep your hands farther from your eyes on the screen compared to a laptop, but that probably just takes a few months to adjust.
Pickit3. I learned a lot... But why didn't I start with Arduino?
RFM non-LoRa Arduinos. Nobody uses these, it's like $5 of cost savings. My custom protocol I built to do error correction with them was incredibly time consuming and not that much fun.
A loft bed and mattress. I could have probably just found a more durable cot really. The extra storage space didn't help that much, I would have been better off just.. buying less stuff.
Any brushed motor tool when brushless is available used for the same price as the brushed one new and on sale.
Earbuds. I lost the charging case on one, and an earbud on another. Not doing that again, I'll just use normal Bluetooth headphones or else not do headphones at all.
12v gear. Everyone who starts with electronics will at some point think about having a single 12v transformer and running their whole workbench on it. They will pick a connector and start building around their new standard.
I chose XT60 which requires a bunch of handmade cables. Such a hassle. Just use USB-C and you can run on 12v easily if you ever need to.
Tons of random parts and pieces, to the point where I'm trying to put together an Awesome List in celebration of all the super common parts and materials which are all you need for 99% of stuff.
Any product containing lead. I'm sure the amount of exposure was trivial but really there was just no reason to be messing with it when there are alternatives.
A catalytic butane heat gun. The one I have is just kind of OK.
A Hakko soldering iron. Those FX-888 type ones are so expensive especially for the older tip format.