Wall supply transition to a 2nd wall supply
There should be no issues with reversals.  The circuit may stop to function because there may not be sufficient hold time on teh charge storage on board.  or some of the rails may droop whilst others are held.  This may be more problematic in certain cases with parts of the circuit having gone through a reset whilst other parts are in an old state.  without knowing what the circuit is and it' very hard to tell.  "Hold time", "brown out sensitivity", "rail sag", "Rail droop" or "power droop" to name a few searches.