To reproduce this bug:

1. Open 'uGUI_Bug_Repro' project
2. Open 'Repro' scene which has a button and point light with flare behind it
3. Select Canvas in the inspector
4. Set Main Camera as Render Camera
5. Choose Render Mode 'Screen Space - Camera'
6. Notice lens flare effects rendering in front of Button even though it is behind it
7. The same behavior is achieved with 'World Space' Render Mode

http://screencast.com/t/2MHnyBF9

Works as expected with 'Screen Space - Overlay'