This has happened to me a couple times in the past few months, and since I use IB (where there's no on-screen log of every change you make to an order) and I'm usually juggling several orders at once, I thought I was just hallucinating, but last week I saw it in real-time. I tried to sell a put for .60. Saw the order was sitting at CBOE. I replaced it with an order for .70. Status was green, showing it got successfully replaced. A minute or so later, it got filled at PHLX for .60. (To add insult to injury, a screen dump I took a few seconds later showed PHLX's offer was now up to .85). Is this legit? Can PHLX fill an order for 2 minutes after it's been submitted even if it's been cancel-replaced?