I have had a problem with doors disappearing. Passive mobs would get out of their rooms or I would suddenly find a creeper in my home. I noticed it was always certain doors and eventually figured out why: any door I had mounted to only glass blocks would occasionally disappear when loading the world.  To make it easier to spy nearby hostile mobs when I leave a building I will often frame a door in all glass blocks so I can see any waiting near that not at the door. I had one door that was never disappearing one where I had mounted a storage crate next to the door because of limited space in the small building. Since figuring this out I’ve made it a habit to alway make one lower block on the hinge of a door a crate or a plank and it seems to be promising. 
doors had been more likely to disappear when upgrading from one Minecraft beta version to the next but I’m not sure that’s the only time they did, I didn’t always notice right away.

steps to produce

1. Construct a wall made of glass, at least 3x3. (A 3x3 may not be Cesar you but that’s what I usually use)
2. leave a 2x1 opening in center of wall
3. pick a wooden door and place it over this opening
4. save the world
5. At some later play session load the world. (After migrating to new world seems to be most affected.)
6. examine wall

observation

door is now missing allowing mobs to roam freely through wall. A replacement must be crafted.

expectations

door does not disappear