Redbull Gives You Gas
There was a deal at my local gas station where three cans of Redbull were $7. There was simultaneously a deal where every can of Redbull you bought got you 5c per litre off gas, up to 50 litres. Gas was $2.10 a litre at the time.
I figured out that I could buy 42 cans of Redbull for $98, and get $105 of gas for free. I did this multiple times, and now hate Redbull as a result.
Story credit: Reddit / Joslo88