strange one-way collision

There is nothing hidden

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If IGNORE_CONTENT is not set, a precise collision is done with all nearby passable map and terrain entities, no matter whether IGNORE_MAPS, IGNORE_PASSENT or IGNORE_WORLD is set. At this point in_passable, on_passable and passable_ent are updated to point to the current entity if the trace starting point lies within the entity's axis-aligned bounding box. For terrains the bounding box is assumed to extend all the way down to the bottom of the level. If there is no passable entity at the starting point, but one can be found along the trace line, then passable_ent will be set to this one instead and on_passable gets set as well.

Once all entities have been processed in this way the level blocks get checked. To disable this check IGNORE_WORLD needs to be set.

When this is done a thorough (i.e. slow) test is performed between the requested bounding shapes (ray, polygons, OBB, or ellipsoid). All intersections are stored and sorted by distance from the trace origin. At this point contacts with pushable and passable entities are discarded. The closest hit (if any) is returned.