Some ECN dont route orders to another market centers. Generally brokers dont allow orders worst than bid ask price, but is possible ?
DEVIL IS IN THE DETAILS -flashing quotes, fast market declared, at the opening just to name a few. You may also be owed a trade correction depending on the circumstances. There is also a reg. saying if you make lit markets - your dark markets need to reflect those. The devil is still in the details.
Regulation National Market System (or Reg-NMS) offers protection from trade throughs. The rule is not absolute but is in place during regular trading hours with some exceptions. https://www.nasdaq.com/articles/reg-nms-dummies-2019-05-09
Yes, it happens all the time. I make hundreds trades with stocks and options daily and see stable non fast moving stocks and options pass by the Bid x Ask. I use to keep in touch with ARCA founder at the beginning because of running into all types of issues or locked-markets because we wanted the rebate or complaints the ECN went above several layers.