A social media experiment has backfired for Coles, exposing the supermarket to a flood of negative comments on Twitter.

The supermarket is the latest company to have a social media marketing exercise go terribly wrong, following blunders from Qantas and Coca-Cola.

Didn't see it coming ... Coles later admitted the tweet was a mistake.

The official Coles account last night urged followers to complete the sentence "in my house it's a crime not to buy....."

But the PR exercise quickly fizzled as Twitter users inundated the supermarket's account with negative comments.