Film loses some, but not all, of its sensitivity during development. If you expose it to light while any residual developer remains, it will fog - maybe not a lot, but some. Even if development is well stopped and the film is rinsed, exposure to light will darken the remaining silver, which is similar to fogging. That darkening is silver metal, and it will not be removed by the fixer. If the negative is on a reel or in some sort of holder that cast shadows, the fogging will form a pattern on the film. While turning on the lights after a stop bath may sort of work, it is much better to keep the film in the dark until it is fully fixed.