Reproduction Steps:
1. Import attached project.
2. Open "Test" scene.
3. Observe console.

Result:
When a canvas is created with these elements:
*Canvas
**Canvas
***Panel
**Panel
An error appears within the console:
Assertion failed: Assertion failed on expression: 'outputVertexCount == sharedData.vertexCount'

Note: Error disappears in Play mode, once at least one of the Panels is deactivated. Otherwise, it will remain active in Play mode.

Reproducible in: 2017.1.0b1
Not reproducible in: 5.5.3p1, 5.6.0p1
Regression introduced in: 2017.1.0a3