Your product works perfectly in every demo. QA signs off. The client loves it. Then real usage hits, dozens or hundreds of people touching the system at the same time, and suddenly things that never broke start breaking. This is one of the most common gaps between "it works" and "it works in production," and it has nothing to do with code quality in the way most people assume.
A few things tend to go wrong at once. Your application needs a connection to the database for every request, similar to checkout counters at a shop. If the app hasn't been built to reuse those connections efficiently, you can run out of counters fast, even though the database itself is barely working underneath. The usual reaction is to raise the connection limit. Sometimes that helps. I've seen it just as often just let more people into an already crowded room without fixing why the room filled up in the first place.
The second issue is timing. If two customers try to change the same record at once, a payment, a booking, an account balance, the database has to make one wait. That's normal and correct. It becomes a problem when one operation holds things up far longer than it should, maybe because it's doing unrelated work, calling an external service, generating a document, while still holding the database open. Everyone else queues behind it, and it looks like "the database is slow" when really one operation just hasn't let go. I'd rather see a transaction kept small and focused than watch it sit open while the app waits on an SMS provider on another continent.
There's a sharper version of this too: two processes can each end up waiting on something the other one is holding, and neither can move. Databases are generally smart enough to notice and cancel one of them rather than let both stall forever, but the application still has to know what to do next, which usually means retrying safely. Without that handling built in, a customer just sees an error from a situation the system could have recovered from on its own.
The costly version of all this shows up in exactly the places you'd expect: checkout, bookings, approvals, anything with a limited quantity. If your product ever sells the last available seat to two customers at once, that's this problem, not a coincidence. And it's worth remembering the trouble isn't always a customer at all. A nightly cleanup job, a large report, a bulk import running during business hours can quietly compete with ordinary traffic and slow everyone down without anyone realizing why.
The reason this rarely gets caught before launch is simple: nobody tests with 300 simultaneous users until 300 real users show up. It has to be tested deliberately, on the specific flows where money, inventory, or commitments change hands, not just tested for correctness with one user at a time.
Ask your team: Which of our busiest actions, checkout, booking, payments, have we actually load tested with real concurrent traffic, not just individually?
If the honest answer is "none of them," that's a controlled, well timed engineering task, not a five alarm fire. It's far cheaper to find this on a Tuesday than to have your customers find it for you on launch day.