I put a repeater next to the light sensor block, on its north (f=2) side, that was not pointing out of the block. I thought it should be normally placed like on other sides.
When I placed it on the north side of the block, the repeater got a 1 tick pulse.
1. place a light sensor block
2. place a repeater on its NORTH side, that is NOT pointing out of it
3. the repeater will get a phantom signal