In Icehouse, if you move a piece that has already been played while trying to place or remove another piece, you are said to have crashed. The definition of "move" depends on what type of game you are playing.
Tournament games tend to use a very strict definition. Any change in the footprint of a piece, no matter how small, constitutes a move. If a piece is moved, but it falls back into the same, exact footprint, it is not a move but a jiggle.
Informal games use a lax definition that basically only calls something that changes the strategic location of a piece a move. That is, unless it changes the target of an attack or opens up a fortress, don't worry about it.
The penalty for a crash in Icehouse is to give away the piece you were trying to place or remove. After the penalty, all players attempt to put the pieces back where they were before the game continues.