ok, i dont remember if this is exactly what i was doing but this causes a crash. i've had a couple of similar crashes in other levels when combining 3 blocks, usually involving gravity/fans
In the situation below the three skulls wouldn't match. As the fan is still blowing on the bottom right skull when you take the top skull away the bottom right skull would be rising as the middle skull is falling, meaning the middle and right skull should match before the middle one can fall far enough to reach the leftmost one.
What should happen is the two skulls should match and the remaining one on the left should start flashing and show a message letting you know the level is now unsolvable. I'll test some scenarios including the one shown and see if it crashes for me.