In theory once they fail to deliver at T+2 days a clock starts on them that will force them to buy in in some number of days.
This has been abused though by a scheme that is like check kiting. They will push the uncovered FTD back and forth thus resetting the clock.
On the original point you assert. how does somebody make a sale and the other party not know they did not get the shares delivered? Sounds kind of far fetched to me.