I'm consistently amazed how many developers and security professionals don't have a clear understanding how PPK even works conceptually.

Things like deploying dev keys to various production environments, instead of generating/registering them within said environment.

One of the worst recent security examples... You can't get this data over HTTPS from $OtherAgency, it's "not secure" ... then their suggestion is a "secure" read-only account to the other agency's SQL server (which uses the same TLS 1.3 as HTTPS). This is from person in charge of digital security for a government org.

Or when the security team at some other company emails you their private key.