The EventSystem is updated in Update, after FixedUpdate. So if you have world space buttons that are moved in FixedUpdate (attached to a vehicle moved by physics etc) and a camera with its position updated in Update or LateUpdate etc then the raycasts from the mouse clicks will miss the buttons on most frames.

Steps to reproduce:
1. Open User's attached project
2. Play scene "Test"
3. Try to push the button

Expected result: Button click is registered and a message appears in the Console.
Actual result: Button click is missed.

Reproduced with: 2017.1.0b1, 5.6.0f3, 5.5.3f1, 5.4.5f1

Note: If FixedUpdate is changed to Update, void ButtonClicked() catches button click.