Reproduction steps:
1. Open project attached (RTTest.zip)
2. Open the RBTest scene
3. Hit play
3. Detach game window and try to resize it. The original image should be rendered in it's original resolution in the center of the screen, while the other part of the screen should be clear black.
4. Lock (Win+L) and Unlock the PC (do not turn off play-mode). You can also make Unity to lose the device context any other way.

Expected result: Game view will remain the same, there will be no errors in console.
Actual result: Game view becomes black, appears error: "Releasing render texture whose render buffer is set as Camera's target buffer with Camera.SetTargetBuffers!"

Note: This problem only appears on Direct3D9, when using Camera.SetTargetBuffers().
The same problem persists if the game is built as a standalone windows binary, but the same error and screen blackness comes up even on window resize. It seems, Unity reinitializes device context on each window resize.

Reproduced on: 5.3.6p5, 5.4.4p4, 5.5.3p1, 5.6.0f3, 2017.1.0b1.