According to the spec it throws in the following cases:

  Parent - Moved Node
  disconnect - connected  
  connected (Doc A) - connected (Doc B)  
  connected - disconnected

The last one is the most concerning one. Most of the top performing frameworks do child node reconciliation something like this https://github.com/WebReflection/udomdiff/blob/e58db3ad28b72.... Each node could be unmoved (no-op), new (disconnected), or moved from within the parent (connected). Now if one wants to leverage moveBefore each of these nodes needs to be checked for it's connection state.