What does it mean when a flow trade has been crossed out in red?

A trade that is crossed out in red or struck-out has been nullified/cancelled

You may have noticed some flow trades have a red line running across the entire line of data.

Per the SEC:

"the nullification or modification of transactions may be necessary for the maintenance of a fair and orderly market or the protection of investors and the public interest exist".

The SEC cites "verifiable systems disruptions and malfunctions" as a condition in which designated Trade Officials (employees/officers of the respective exchange) may act to review potentially erroneous transactions. This action must be initiated within 60 minutes of the original transaction.  

Approximately 250 - 500 trades are nullified/modified daily. This is a normal occurrence.

Highlighted in yellow is a UMC options trade that was 'nullified or modified'.

Trades that have been reported as 'nullified' (cancelled) or 'modified' will be crossed out in red at the time that the nullification has been reported. Until then the trade will appear on the flow feed normally.

Trades that are 'modified' will be crossed out in red, with the resulting modification reported to the tape at the time it takes place. The resulting modified trade will (unfortunately) not have any labels to distinguish it as such.

The UMC options trade we highlighted earlier was likely modified, as you can see another UMC trade a couple lines above. Note that the contract sizing is vastly different than the original 'modified' trade.