1) What happened
[ExecuteInEditMode] gets superfluous calls to OnEnable/OnDisable when entering / exiting play mode

2) How can we reproduce it using the example you attached.
Open bugdemo.unity.
Look at the console. this tracks calls to OnEnable / OnDisable.

When entering play mode the script should get disabled, then enabled - but this happens twice.

The resulting error message from the RenderTexture system is an example of how this bug makes it very hard to work with system resources because of this bug.