I at one point, spent some money to get a miniature laptop for traveling uses. And I made the mistake of formatting the storage on it, not realizing I needed to safely partition off the operating system, and ended up trashing the mini laptop due to it.
I generally do a reasonable amount of research before parting with money so I don't recall buying anything that I've actually come to regret but I recently bought something that escalated in price to such an extent it's worthy tale to tell.
I own a vintage Fender Telecaster guitar which has a switch tip (plastic nob) that is notorious for flying off into oblivion. Long story short I recently found a supplier in the US (I'm in the UK) that had a replacement, a replica plastic nob with a screw to stop the thing flying off.
I bought two @ $27.99 each. What I didn't know is there was a $110 shipping charge which triggered customs charges and a massive UK Post Office handling fee. The total charge for two plastic nobs and a couple of screws... $210.95 🙄