A seven speed cassette for my eight speed bicycle. I had assumed it was seven when I got it and just hadn't realised it was eight speed despite riding it for some months and it having an '8' clearly printed on the alivio shifter pod thingy. I didn't realise until I slid the shiny new cassette onto the freehub and noticed the gap.
'........oh... my bike's got 8 gears..doh.'
A radio control 4WD nitro truck....at a time when i was desperate for something to interest me.
£550......1 summers serious use, 1 summers occasional use and its sat in my cupboard ever since, reminding me i really brought it for the wrong reasons
But...the first year gave me some interest and enjoyment, so not quite a complete waste.
I once bought something called a "Backswing" which allows you to flip yourself inverted (because, she asserted, it gives me a heavenly feeling), it should have cost £200 but I just left it at the next digs I was in without paying for it.
Used it once. Froze my nuts off setting it up in the garden in the dark with the instructions in one hand, spent ages trying to train it on the moon (which I could already see just by looking up). Saw that it was a bit bigger but still round and white. Then took the whole thing apart again and stuck it in the loft.