Paintings and Item Frames can overlap.
What I expected to happen was...:
When trying to place a Painting on an Item Frame, it doesn't get placed or both things drop as item.
What actually happened was...:
The Painting gets placed on the Item Frame without Problems.
Steps to Reproduce:
1. Build an 1x1 Wall
2. Place Item Frame on the Wall
3. Place Painting on the Wall
Note: You need to aim at the Top/Bottom Left/Right Corner to place it.
4. A Painting and an Item Frame are in the same space
The thing is, item frames can stick everywhere, paintings aren't the problem. You can stick item frames behind buttons, levers, torches, everything.