We hit exactly this kind of race condition in our Go + Postgres SaaS when handling concurrent waitlist signups. Two requests would read the current count, both pass the limit check, and both insert — exceeding the waitlist cap.
Ended up using SELECT FOR UPDATE on the waitlist row before the count check. Simple but effective. The barrier testing approach described here would have caught this much earlier in development instead of discovering it under load.
One thing I'd add: in Go, it's tempting to handle this at the application level with mutexes, but that breaks the moment you have multiple instances. Pushing the serialization down to Postgres is almost always the right call for correctness.