Minecart driving up slope from the side loses too little momentum

When a minecart drives from a regular rail sideways onto a diagonal rail, it goes up that rail (probably intended) and loses a bit of momentum, but not enough. That way it gains more momentum going down that diagonal rail than it lost before.

This can be used for a really bad booster and was shown in one of Etho's first Let's Play episodes, back in Minecraft beta.

Video: https://youtu.be/ilO26HFieS8

The underlying problem is probably the same as MC-2714: Minecarts snap to rails instantly, without changing their momentum or considering blocks in the way.