When I built my coop and run I:
1. didn't make the fence high enough. When the birds were flying out, I had to add height to my fence. I think a 6' tall fence is appropriate for most chickens. Mine was only 4'.
2. didn't put in enough ventilation. Had to modify the coop to add more.
3. Here's one I've never seen mentioned. My coop, at the lowest wall (where the pop door is) is about 5' 4" higher than the ground. Part of this wall is inside the run. Today one of my hens was found in the neighbor's yard. I think she flew up on the roof (5' 4" off the ground), walked to the other side, then made her escape out of the run. Good thing I have good neighbors.
Fencing has been added to eliminate the possibility of this happening.
Ed
1. didn't make the fence high enough. When the birds were flying out, I had to add height to my fence. I think a 6' tall fence is appropriate for most chickens. Mine was only 4'.
2. didn't put in enough ventilation. Had to modify the coop to add more.
3. Here's one I've never seen mentioned. My coop, at the lowest wall (where the pop door is) is about 5' 4" higher than the ground. Part of this wall is inside the run. Today one of my hens was found in the neighbor's yard. I think she flew up on the roof (5' 4" off the ground), walked to the other side, then made her escape out of the run. Good thing I have good neighbors.

Ed