it seems based on the facts the execution price was too far away from the last price to be accepted. this is regardless of the actions by the OP even while including the possibility of a fat finger error.
I realized that I have access to time and sales for BCOR due to a data subscription and was able to pull up afterhours trading for BCOR. You are correct, the best bid at the time was $6.40 and your order hit the market priced at $8.85. According to my time and sales only 4,300 shares were traded there. There was no extra 1,000 shares traded at that price. Either you accidentally left off the 1 from $18.85, or your broker has a weird default pricing mechanism or incredibly serious software bug. Regardless of how the $8.85 came to be, it seems like your best bet is to pursue your broker for negligence in more than one way.
Note that it is possible that I am not seeing the extra 1,000 shares traded at $8.85 because it was busted and removed from my data source.
I changed the quantity of my order to hit the bid of 4300 shares at $18.85. That's what I could read on my screen but in the past my broker told me that it ever happened that the bid or offer would not update to show the reality of the market. That would be great if I could find where the bid was if there was really one.
It could be very helpful if you have a log file from your trading software that shows the exact details of the order line which was sent from your computer to your broker. Until now you keep on repeating "I saw this... I saw that ...". Does your trading software keep a log file on your computer?
I think that regardless of the mistake about the price, the problem is the fact that my broker failed to contact ARCA immediately.