If a Minecart goes up a Hill, in a setup like this:
.....R
RRD
DDD
D being Dirt Block and R being Rail, it will quickly face the wrong direction by 90 degrees if on the topmost "Top of the Mountain" Rail Block before being correct again.
This is noticeable on both, when going up and down the hill, also noticeable in first person when inside the Minecart, although there it feels like the rotation is only 45 degrees.
Oddly enough, it looks like Minecarts turn almost 180 degrees after going off track.

This seems to happen with all minecart types, the almost 180 spin when off track also is best noticeable with Furnace and Chest Minecarts due to them having a clear front/rear section.