A client is like: Help! We got a 500 in our wordpress administration panel and there is no error in the log, it must be your infrastructure at fault!

So I calmly replied to them that wordpress handles its errors on its own, and without the appropriate debug flags enabled, doesn't log it anywhere. Even mentioned that a PHP app can change the error handler no problem, and linked them to both, PHP and Word press docummentation.

Didn't hear from them since.

@JustThat not really, I deal with clients that pay for the managed server tier, paying a constant amount whether they asked for support or not.
But we only provide support to the system and services. Client apps are completely outside of our scope of responsibilities.
I could have just answered: It's an error in your app, we do not provide support to such errors.
But being pro-client, I invested the little time to explain why the error wasn't logged, and pointed them to appropriate documents to solve the issue next time.

define('WP_DEBUG', true);