As far as I know a broker can not execute you at a price that has not been reached during the day. Look at the High and the Low in a source like Bloomberg or Reuters. If your exec price is above/below that high/low, then there is a problem.
It happend to me with one of our broker for our fund. They sold us GBP at a price that was above the market... then they re-priced the transaction.
Looks like Oanda had some problems yesterday. Anyway they have 2 pips spread, that's great, but they have problems with stops and orders.... so I moved to another broker. Stilll using Oanda in
Demo to test another system in parrallel.