Reproduction steps:
1. Open attached project "ReadPixelsError"
2. Build scene "Test1" for iOS
3. Run the game on a device or through a simulator
4. Look in the Xcode log
--- You should see this error: ReadPixels was called to read pixels from system frame buffer, while not inside drawing frame

Code that seems to be causing this issue:
IEnumerator Start () {
...
yield return new WaitForEndOfFrame();
targetTexture.ReadPixels(new Rect(0, 0, renderTexture.width, renderTexture.height), 0, 0);
targetTexture.Apply();
...
}

Notes:
--- No error in editor nor in OSX Standalone
--- On iOS error is thrown with both IL2CPP and Mono

Regression since version: 5.2.1p3 (b7d18ab613bc)