A: Common errors include authentication failures (wrong username/password), “NetworkTimeout” if Atlas is unreachable, “Server selection timeout” if the cluster can’t be selected, or DNS/hostname resolution errors. To debug, ensure your connection string is correct, credentials are valid, IP whitelist is configured in Atlas, and network connectivity (firewalls) is open. Logging the error object (e.g. in the catch) often gives a helpful message. Also, listening to connection events (error, disconnected, reconnected) on mongoose.connection helps track status.
