I have unscrewed a 2 gang light switch during repainting of my living room. I noticed that one of the black wires had come off and I put it back to where I thought it should be (L2, right switch on the diagram).
The lights are now working, however, one of the switches is now interfering with the light in the adjacent room that will not turn on when one of the switches is on. I examined the wiring more carefully and used a tester to check which ones are live. I found only one live (red) cable that was connected to both COM ports. The second red was connected to the L1 port and is not live. I am obviously missing something here, but I can't figure it out.