When one uses hash tags out of context in social networks. Mainly used in comments on Instragram, hashbombing is similar to photobombing, in that it draws attention away from the photographers original intention.
Chad: Why the fuck did Spencer comment #dogcrap on that picture of my grilled cheese sandwhich?
Al: "Dude you just got hashbombed. Now your grilled cheese is going to show up with pictures of dog doo."