To reproduce:
1. Open attached Unity project "PolygonCollider2D Diagonal Test.zip"
1. Play "test" scene.
2. Hold horizontal input axis keys to move the box
3. Move to the corner of the letter "O"

Expected result: https://gfycat.com/VeneratedAdolescentGrayling
Actual result: https://gfycat.com/GlamorousEcstaticJumpingbean

Reproduced on: 5.5.0f3, 5.6.0f3, 2017.1.0b1, 5.6.0p3

Note: the bug only occurs if the player is moving very slowly at the time of contact with the corner of the letter "O" (increasing the timescale variable
in "PlayerController" script will eliminate the bug)
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Multiple polygons whether they be separate colliders or the multiple convex polygons that PolygonCollider2D produces do not create a continuous surface and the physics system handles contacts with each polygon separately. This can lead to "catching" corners. Increasing the contact offset can help with this sometimes but not always. Only the EdgeCollider2D or the CompositeCollider2D (using Outline geometry) can produce continuous surfaces.