Duplicating Horses.
What I expected to happen was...:
I expected to return to my house on my house, and I hoped that nothing would happen.
What actually happened was...:
When I got to my house, I found another horse in the exact same place where I left off. The amazing part is that it had the saddle and horse armor on it as well.
Steps to Reproduce:
1. Make a pen for a horse (I had no leads.).
2. Ride your horse far enough away so that your house is out of render distance.
3. Come back and see the cloned horse.
(DISCLAIMER: I am not even sure if this explanation works, but it is how it happened. Also, this has happened more than once.)
Related issues
Comments
1. Please update to 1.6.2 and see if it is still a problem.
2. Was this on a server or in a singleplayer world?

Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
Note: this is a automatic message, so apologies if I miss something.
It was on a singleplayer world.
The only odd thing is that when I logged out to write this bug log, when I logged in, The extra horse disappeared..
Can you reproduce this or was it a one-time thing?
My horse just duplicated yesterday in 1.6.2. I cannot reproduce it, but it happened. The horse was the same colour, had the same armour, same speed, jump and health, etc. I'm not sure how it happened, but a horse duplication bug still exists in 1.6.2.
The next time I logged, The extra horse somehow went away.
Derp.