I started playing in survival single player a few days ago, after having not played since 1.5
Built a small house and a stable in a large mesa biome, then went to go tame a horse
Brought the horse back to the stable. Before quitting for the day, I took a short ride around the edges of the mesa biome, then returned to the stable and closed the horse into it's stall (no possibility of it being able to escape the way I constructed it).
Upon relogging, the horse and its saddle had disappeared. The few chunks around the stable had also reset- the hay bale I had put into the stable stall disappeared, the fence gates were all open (they had been closed), a bed I had put down had disappeared, and items that I had smelted and collected were back in the furnace. Inventory remained unchanged; I had smelted 16 stone and made (and placed) 2 pressure plates (12 stone left)- upon relogging, I had the 12 stone in my inventory, no pressure plates, and another 16 stone back in the furnace.
I decided to test again, taming another horse and bringing it back to the stable, but taking the saddle off this time. After relogging, the horse was again gone (I can't tell if the chunks had reset as I hadn't put down any blocks this time).
I go again to catch a third horse, bring it back, then quit while on the horse- at this point the horse was still around after relogging. About an hour after that, I had taken the horse for a brief ride, came back to the stable, and closed all the gates. After relogging, the gates were all open and the horse was gone again. However, a bridge I had constructed 5 minutes prior to taking the horse for a ride (about 2 or 3 chunks away) was still there.
Although this may be related to similar 'duplicate' bugs, all those bugs are resolved, while my bug is ongoing.
I recall having a similar bug around the Adventure Update, but that bug did not save any progress made in the entire world, while this current bug only seems to affect a certain number of chunks.