Can a Varnish cache error cause PayPal transactions to fail, and if so, how?

Yes, a Varnish cache error can absolutely cause PayPal transactions to fail. If Varnish is unable to serve the correct PayPal response (due to an error, a stale cache entry, or a network issue), the transaction will likely be rejected by PayPal. This can result in declined payments or delayed processing. The failure often stems from Varnish not being able to retrieve a valid response from the PayPal API, forcing it to return an error to the client.