Reproduction steps:
1. Open my attached project ("case_858816 repro")
2. Make sure that Interpolate under RigidBody is set either to 'Interpolate' or 'Extrapolate'
3. Enter Play mode
4. Move the Cube with 'w' and 's' keys

Expected: small position errors due to Interpolation
Actual: the position error between transform.position and rigidbody.position is getting bigger and bigger, depending on how far away it is from the starting position.

Reproducible on: 5.5.0f3, 5.6.0a6
Not reproducible on: 5.3.7p1, 5.4.3p3