As an ex-operator I can tell you how it used to work (and...

As an ex-operator I can tell you how it used to work (and probably still does).

I remember when a female operator put a sell order in for 2 million BIL @ 30c (30 instead of 3000) before market open. Everyone except her saw it but it wasn't removed. Market opend @ 8.00, then shot straight back to around previous days close (32.00)

If an operator makes a mistake such as this, the general consensus is that the trade is reversed (the reasoning being that you would expect the same if you stuffed up). Some operators (esp. with online brokers) would not reverse the trade so heaven help them if they made a mistake (their rational was that they had STP so they had already reported to the client the buy).

The ASX also may step in and reverse trades where it feels that the market was 'tainted' (for use of a better word- can't remember the phrase that was used). If it only dropped the stock 10% etc, you would find the ASX would not worry about it- bad luck.

And before you think that it is a hard thing to do- you hit sell 1m .085 and don't realise that you haven't changed the code, override the warnings and bam! spend the next hour on the phone cancelling trades and the next month buying beers for the operators that cancelled them.

As for the BIL operator, not all the trades were cancelled, she lost the broker $$$ and I think she was not required the next day.

Here endeth the reminiscing...