In fact, I believe in the united states, with an ECN they are permitted to revert trades which are mistaken, while dealing desks are not.
It's the new rule.
Http://www.nfa.futures.org/NFA-faqs/...e-2-43-QA.HTML
CR 2-43(a), Price Adjustments1
Q. Section (a)(1)(I) of this rule provides an exception from the prohibition on price alterations in which the alteration is advantageous to the customer and is done as part of their settlement of a customer complaint. Does that imply a Forex Dealer Member (FDM) can not make a favorable adjustment if the customer does not complain?
A. It depends upon the conditions. The intent of this provision is to make sure that FDMs can repay customer complaints after they wind up in mediation. It wasn't meant to prohibit FDMs from adjusting prices on customer orders which were adversely affected by a glitch from the FDM's platform. A firm may not, however, fix prices on customer orders which benefited from the error (except as provided in part (a)(1)(ii)). Furthermore, an FDM may not cherry-pick which accounts to correct.
Q. An FDM operates several trading platforms. Two provide only straight-through processing, but one does not. Can the FDM make part (a)(1)(ii) alterations for trades placed on the two platforms which provide straight-through processing?
A. No. The Board meant to restrict the aid of all those companies that only operate a straight-through processing business model, and the submission letter to the CFTC uses this speech when describing the rule's intent. NFA recognizes, however, that the use of this word platform in the rule itself may be perplexing, and we mean to ask the Board to eliminate that word at its August meeting.
Q. For price alterations made under section (a)(1)(ii), the rule requires written notifiion to clients within fifteen minutes. In case the liquidity provider informs an FDM of the price change twenty minutes after the orders are implemented, can the FDM still make the adjustment?
A. No. The rule provides that clients must be notified within fifteen minutes after their orders are executed, and it had been written that way intentionally. Since a client's subsequent trading decisions may be based on the client's belief that a specific trade was implemented at a particular price, the rule stipulates a narrow window to get price alterations.
1 For purposes of this discussion, the term adjustment also pertains to cancellations.
Pretty secure at an STP also, for that matter.