I just found Ger-sey Girl's blog recently so I've been catching up. In one post, she admits to mistaking vinegar for vegetable oil, and even cooking with it a few times before realizing her mistake!
When we first moved here, we only had a few weeks of evening German classes under our belts and knew very little. Pictures and packaging were more important than ever in trying to figure out what we were buying! "This bottle is shaped like a conditioner bottle....hmm....well, hopefully it's conditioner." "Is this laundry detergent? Well, in the diagrams on the back, it looks like they're putting it in a washing machine. So....maybe?"
Amazingly, no mistakes were made during this period! We managed to buy the right things and soon learned all the words so that wasn't a problem anymore. No, my big embarrassing mistake was just based on laziness - I was in a hurry, they were next to what I really did want, and I didn't read the package - and wasn't very long ago. Hopefully the cashier assumed I was buying the incontinence pads for someone else. Gah!!!