While I was traveling in space in a ship with two XS space fuel tanks, one of them ran empty while the other was half-full (the engines were not taking from this tank). My ship couldn't travel further, so I emptied the half-full tank and attempted to fill the empty one.
I got a message saying that the fuel is incompatible, even though I'm sure that the fuel used for that tank is correct.
I think it's a bug with the space fuel tanks....