Magnus effect is a principle, discovered by Heinrich Gustav Magnus, which creates a tangential force on a moving rotating body due to its interaction with the air. The story says that Magnus discovered this phenomenon on cylindrical bodies and that other physicists studied it on different shapes. In any case, in ballistics, we call this phenomenon "Magnus effect", which tends to make the trajectories go out of the shooting plane.
In reality, this effect does not have an important direct impact on trajectories, at least for small projectiles. On the other hand, Magnus effect tends to accentuate the obliquity of the projectile and to indirectly disturb its trajectory.