2005 Prius went totally dead, but came back

A loose clamp could have caused it, but no one knows what other reasons might have caused it.

 

Was the connector all the way down on the post? I don't recall if this is the case for the Prius 12V, but at least some battery terminals are not cylindrical, instead they are slightly conical (base part of the cone, the pointy part is missing). If the connector is not shoved down all the way to the deck of the battery on one of those the connector "loop" may close fully without getting a very good grip on the post and then it could work loose.

Also, I'm not sure if this is what you meant, but I never put dielectric grease between the post and the connector. The grease is there to prevent corrosion, it goes over the top of everything once assembled to keep water and air away from the metal. If there is grease between the post and the connector it is lubing a junction which is not supposed to move, which is counterproductive.

 

All automotive battery terminals are conical as you describe. That is why it is important to open up the clamp before reattaching it and ensuring the clamp is firmly seated all the way to the bottom of the post.

Remembering to first loosen the clamp and force it open a bit before attaching and tightening.

I 100% agree. I do not use any corrosion countermeasure and the battery posts are 100% corrosion-free, unlike the (good) old days of unsealed batteries.

 

There is a paste that's supposed to prevent corrosion in battery terminals.