Range lights and the two-second habit
A pair of range lights is the simplest piece of navigation equipment ever built and the easiest to use badly. The rule is that when the rear light sits directly above the front light you are on the channel centreline. What people get wrong is treating it as a check rather than a control: you look, you correct, you look again two seconds later. Glancing once, deciding you are close enough and turning your attention elsewhere is how a boat ends up fifteen metres off a leading line that it was, briefly, exactly on.