Common Sense - Proper implemention any security technology/product will provide the expected results AND more importantly eliminate the "false sense of security".

Here is an example of incorrect implementation that lead to eFail vulnerabilities - These vulnerabilities could allow potential attackers to decrypt the content of your end-to-end encrypted emails in plaintext, even for messages sent in the past. 

The flaw doesn't reside in the email encryption standards itself; instead, it affects a few email clients/plugins that incorrectly implemented the technologies.

https://thehackernews.com/2018/05/efail-pgp-email-encryption.html