Aaargghhh!
Picked up what I thought were Red Leicester slices to go on some Ritz crackers. Have got... vegan cheese. Gluten-free, milk-free, made with coconut oil. Unless any vegans can tell me it tastes different melted, all I can say is yuk, double yuk, eww and IACGMOOH 
I tend to buy the wrong thing when not concentrating or when products are together but so close I pick up Korma instead of Massala for example.
Can't think of any mistake that has ever ended well and introduced me to something awesome.
Please cheer me up with mistakes that were more costly or vile than mine (two quid down the drain, thought it was dear). Or alternatively any wonderful breakthrough/serendipitous discoveries 