I can't think of any real howlers.
In terms of (lack of) VFM it might be my Cube eMTB. It cost me £3k, 3 years ago and I've done about 300 miles on it! I just underestimated how much off road riding I'd do! It's a lovely bike but cost per mile has been way more than my car!
Weird thing is though I love it so much I can't regret it.
This is very little money in the scheme of things but the one I feel worst about was my son's bike. Apollo kid's single speed bike brand new from
Halfords and stabilisers fitted, paid for a 2 year service plan (yeah I didn't know any better). Son rode it twice, once up and down the drive while I set the saddle height, then we took it to the local park. He was just enjoying himself and getting some confidence and then he fell off. He's not sat on it since then and is not interested at all. 😢 I should get rid, it's only taking up bike space in the garage.