[piemonkey]

There are a number of interesting examples in computer vision and psychology literature which document difficult cases for visual perception, see for example slides 14-19 in [0]. Ultimately, vision is a holistic process with edge cases that require knowledge of physics, human psychology, and so-called "common-sense reasoning" in order to resolve. Depending on how one phrases the objective of what a computer vision system should do, the problem can go from a tractable subset of automated reasoning to an intractable general AI task, often with very subtle changes to the problem statement.

[0] http://www.cs.toronto.edu/~urtasun/courses/CV/lecture01.pdf