You're right about combat testing, but there are some limitations with the basic RPGMaker (I'm on MV) combat editor - for one, it's not immediately obvious how to test a battle where an actor enters with a status effect!
I think he was talking about poor-planning behaviors like using variables without giving them names and then accidentally using them to track a second thing, or making a section of a game dependent on a resource location the player would already have exhausted long ago in a full playthrough.
There are countless ways to softlock - and even hardlock! - your players without crashing the game. For example: start making a common event that switches on when a certain switch is flicked, and then change what that boolean does later, forgetting it was used for a common event... and you've made a complete monster. I bet with sufficiently bad logic, you could even get a normal event object to crash the game. It's par for the course with anything programming related; there are *always* bizarre edge cases that let the player do something unanticipated.