It's a trap.

Never go to the store hungry. I did that the other day and bought oreos, cheese-its, ramen noodles, and the things that I actually went to the store to buy.

I thought this was the only problem with going to the supermarket until I read the chapter Checking out of Supermarkets in the book Stuffed and Starved by Raj Patel. 

Atmospherics. Ah, there's the problem. We go into the store to get some milk for our cereal at home, but we have to walk all the way to the back just to get it.This gives the supermarket time to trap us with a song making us more comfortable and at ease in the store then with each step we see different things on sale tempting us to buy them. 

Never go to the store. I did that the other day when all I needed was sugar, but naturally by the sugar was the cookie mix and then the song "Pour Some Sugar On Me" by Def Leppard came on and it made me want to shop a little bit longer, like really, who can leave in the middle of that song? Then I decided to have a baking party and bought brownie and cupcake mix as well. I spent $20 instead of $3. Damn.