Steps to repro:

1. Open Unity;
2. Create new project;
3. Go to PlayerSettings (Edit -> Project Settings -> Player);
4. In the Inspector uncheck 'Auto Graphics API for Windows';
5. Click '+' button;
6. Select 'OpenGLES2' graphics API;
7. Drag and drop 'OpenGLES2' platform on top of Direct3d11 to switch graphics API to OpenGLES2;
8. Drag and drop 'Direct3D11' platform on top of OpenGLES2 to switch graphics API to Direct3d11;
9. Observe the console.

Actual result:
'The new context (0000000000020007) must already be the current OpenGL context (DEADDEADDEADDEAD)' error is thrown in the console. See attached video and actual.png screenshot.

Regression introduced in Unity 5.5.0.

Expected result:
Graphics API should be switched without errors.

Notes:
- Reproducible in 5.5.0a6;
- Not reproducible in 5.4.0f3, 5.4.0p2;
- Reproducible on both Windows 10 and OSX 10.11.6;
- Note that bug's reproducibility on OSX and Windows vary depending on command line arguments. On OSX the bug reproduces when switching graphics API from OpenGlCore to Metal only when launching Unity in -force-gfx-direct mode and doesn't repro in general multi-threaded mode. On Windows the bug reproduces only in multi-threaded mode and doesn't repro in -force-gfx-direct.