This scam occurs predominantly in Thailand and India. As travelers find amazing deals online nowadays, shady owners post great pictures and descriptions of their rooms, but when the tourists get there they find that they booked and paid for a room that is either not available anymore or the pictures were far from reality.
In the first case, the victims are required to pay an upgrade since their room was “accidentally” double-booked, while in the second the room is just full of bugs and smells like hell.
How to avoid: always book your trip to poor countries only though reputable online companies. If you think you find an amazing deal, brag about that only after you come back.