In a post on the company's forum, OnePlus One owner MiYzu writes that the phone blew up as he was riding the underground. The explosion burnt the phone's case, put a hole in his jeans and burnt his leg.
OnePlus has already dispatched people to MiYzu's residence to physically retrieve the blown-up phone and found out exactly what happened, according to a statement they issued to Android Police.
The exploding tendencies of lithium-ion batteries are well-known, but according to Android Police, the OnePlus One uses a lithium polymer battery, which should make it more difficult to damage due to external stresses (you know, like sitting on it). [Android Police]
Top image: MiYzu/Android Police