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Yes, unfortunately, there are maps that require this behavior.

Yes, go to map18 of doom2. Shoot to open the door right at the start, then keep shooting through the open door. You can hear the switch clicking as you do so.

I just tested it, and it appears to only be the case with tracers. IE if you shoot a rocket at a 'projectile hits' line and it doesn't hit anything (but just passes over the line) then nothing happens. However if you shoot the same line with a tracer then it executes the line special as if it had actually hit the line.

as Graf said though, this is how original doom did it (note original bullet activated lines did not work with rockets/plasma)...

Hirogen2 wrote:When I shoot a projectile at something, and that (for me it was a pistol bullet) passes a linedef that is set to be activated when the "projectile hits", the line still gets activated when the projectile crosses the line.

That's the way Doom did it. Changing it is not recommended. I know some maps that require this 'bug' (e.g. Memento Mori 2 Map01)

When I shoot a projectile at something, and that (for me it was a pistol bullet) passes a linedef that is set to be activated when the "projectile hits", the line still gets activated when the projectile crosses the line.