Rolling back transactions via transaction listener?

I've been thinking for a while that if a nsITransaction does something 
it's not supposed to do, I'd like to cancel the transaction and undo it 
right away.  Something like:

didDo: function didDo(aTxMgr, aTxn, aResult) {
   if (/* something unexpected */) {
     aTxMgr.undoTransaction();
   }
}

Is there any reason that a transaction listener should not do this?

Alex Vincent