Most managers agree that backing out of a trade is not recommended, but there can be circumstances that warrant such a move.
Managers that are new to PB or have had trouble getting untracked should be given some leeway by more seasoned and successful managers.
It is rather harsh to say that because a manager makes a mistake he should never be trusted to trade with again.

As to PB's policy:
we allow backing out up until I approve the trade---for whatever reason.