I'm on a world where Hardmode has not been activated, and I found a demon altar spawned over some sand. Here is a visual representation:
[*] DDS DDS [*] = Altar D = Dirt S = Sand
I was clearing the path and by mistake I removed the bottom sand block, making the upper one fall, and then the altar wasn't over 3 blocks anymore. The game made a sound like when a zombie dies, and the altar dissapeared.
No message appeared. I didn't lose any HP. It may have any colateral effect in-game?