They call it "ghosting" (or flicker) when a LED glows dimly when it should be off - I think it's called "ghosting" because it's to do with a switch in the same row etc being on that causes it.

I have seen mention of hacked ROM for WPC games to stop this, but as I am not that big on LEDs (other than in certain places) haven't gone much into it. Should be stuff around, just google "Pinball LED ghosting".