Reply #18: trade busts are pretty common.

[Statistical]

However never before (as far as I can remember) on a scale this large.

Usually it is an issue with a single security. Sometimes a security should be halted bur not all Brokers get the word and trading still occurs (directly broker to broker or customers inside brokerage). Usually those trades will be busted.

Also there can be issues with individual stocks related to trading programs, or failures of market makers (market maker system goes down all liquidity in a stock goes to 0).

I have been in busted trades twice and both times I was on the wrong side of it. I wasn't affected yesterday though although I did buy to cover a call spread on SPX near the bottom. Was pretty tough to get price discovery there was no liquidity couple dollars between bid and ask and I had to close both sides of the trade. Hopefully they don't bust that.

Still this is unprecedented in the size and scope of the chaos.