If you advised a friend on cryptography/databases and they started pushing back and repeating some questionable cliches? There's two sides to this. I've heard it's quite frustrating to be a teacher or doctor nowadays...

There is a huge asymmetry in the consequences of not pushing back. While the expert might be frequently frustrated by having to explain away the same cliches, the patient/customer has a lot to lose by not pushing back. Especially when the topic in question is a person's health.

On the flip side, how many programmers still make mistakes in cryptography and databases when they are knowledgeable about them? Could any single programmer write a cryptographically secure 'anything' even as an expert in their field?

Now imagine the program they are writing is someone else's life and they are an expert, but the tools they use are unstable, have frequent revisions and don't notify you when something changes and have tons of undefined behavior. Testing also likes to return more or less random values and you can only kind of tell what the outcome has been.