Since the addition of Armor Stands, one of their features has been their ability to be picked up by Minecarts. It's a subtle feature, but it's been confirmed as intentional by Searge, and is really useful for certain mechanisms like interchangable armories and such.

However, in 1.10, I've noticed this mechanic appears not to work at all. Before, pushing a Minecart into an Armor Stand (or vise versa) would load the Armor Stand into the Minecart, but now the two just collide and don't interact at all.

The two gifs below show how it used to work, and what happens now in 1.10 – I first loaded the world in 1.9.4, then opened it again in 1.10.2 in the second gif. You can see that the Armor Stand from the first gif stayed in the Minecart in 1.10.2 – so it's not a riding NBT issue or anything – it just won't let me load new Armor Stands into new Minecarts for whatever reason.

It'd be awesome if this could get fixed for 1.10.3 or some other minor version before 1.11, since it renders quite a few 1.9 Redstone mechanisms totally unusable...

(Turns out Gifs are too memory heavy to upload, so here's some imgur links.)
1.9.4:
http://imgur.com/WPi8yAe
1.10.2:
http://imgur.com/ZvmncoK