My worst impulse buy was a 64oz container of what I thought was bulk mac and cheese powder from a local big-box store. Was going to use it on popcorn, to cheese up random dishes, etc.
What I got was, indeed, a 64oz container of cheese-flavored powder, but it was more like cheese-flavored salt. I guess it was for seasoning popcorn or something? I tried that, and it was not good on popcorn.
Either way, I used one teaspoon of it and that was it. That stuff was terrible and not at all what I thought I was buying.