Steps to reproduce:
1. Open attached “NavMesh.zip” Unity project
2. Open “Main” scene
3. Enter Play mode
4. Observe Console window

Expected results:
NavMesh.SamplePosition with a specified ‘Walkable’ areaMask returns True
Actual results:
NavMesh.SamplePosition returns False

Reproduced in: 2018.1.0a7, 2017.3.0f1, 2017.2.0p4, 2017.1.2p3, 2017.1.2p3, 5.6.4p3, 5.6.0f1

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By design:
Incorrect areaMask parameter passed to NavMesh.SamplePosition.
Correct usage:
var areaMaskFromName = 1 << NavMesh.GetAreaFromName("Walkable")
found = NavMesh.SamplePosition(transform.position, out result, 10, areaMaskFromName);