This is protection against "user error".

In many Hive applications, a user can send a transfer or some form of custom_json, and sometimes they don't understand the instructions of an app, and accidentally put a private key into their actual transaction. Most commonly this is done when someone puts a private key into a memo field. This is pretty rate, but it does happen, and then everyone can see their private key (hence we call it leaking a private key).

We're designing Wax so that any app that uses it to construct transactions will automatically get a check that detects such leaks and prevents these transactions from being broadcast.