I have seen this happen many times on FXCM, once I discovered it, it is when their unreliable platform ends up, and one facet of the price stinks, ie. Usdjpy ask price stuck at 106.50 while the market kept going up, state the bid price is 106.6 now, then the spread is -10pips, but from my experience the price is NOT tradable, it is going to provide me the order revoked, market moved message, however I didn't attempt it everytime, possibly sometime it'll work, but it will probably get reversed back anyway. .