What actually happens when a trade fails to settle, from someone who spent ten years cleaning up the mess

Ten years in trade support across seven banks. Securities lending, Delta One, prime brokerage, OTC equity swaps. Settlement fails were the background noise of every morning I ever worked. Everyone preps "walk me through the trade lifecycle" for interviews. Almost nobody can answer the follow up: so what happens when settlement doesn't happen?

Here is the actual sequence, because it comes up in interviews and nobody teaches it.

1. The fail shows up the next morning. Your custodian feed says the stock didn't arrive, or didn't leave. Nothing dramatic happens. No alarms. Just a line on a report that somebody has to own.

2. First job is the cause, and there is a standard list. The seller is short of inventory, usually a stock loan recall that came back late. Settlement instructions don't match, one side has the wrong account or the wrong place of settlement. A corporate action has frozen the line. Or you got a partial delivery and the balance is hanging. A good fails person runs that list in order without being told.

3. Then it starts costing money. In Europe, CSDR penalties have been live since 2022. The failing party gets charged daily, basis points on the value of the failing trade, collected through the CSD and passed on monthly. Small numbers per day, real numbers per month. Someone internally has to work out which desk or client eats it. That someone is this seat.

4. If it drags on, it escalates. The buyer can start a buy in, meaning they purchase the stock elsewhere and the failing seller pays the difference. Fails that cross a dividend record date turn into market claims. Fails that cost someone funding turn into interest claims. Every one of those lands on the middle office first.

5. The chasing itself is the job. Calling the counterparty's settlements team, chasing the stock loan desk for the recall, getting SSIs repaired, deciding when to warn the desk that their short is about to get bought in. The traders find out a fail exists when it is already handled or when it is about to hurt. Both of those messages come from you.

Interview version: if someone asks you why a trade might fail and you can walk cause, cost, escalation in that order, you sound like you have sat in the seat. If you recite the lifecycle and stop, you sound like everyone else.

Worked a fail scenario you didn't understand, or prepping for an interview where this might come up? Ask below and I'll go through it.