func_nav_avoidance_obstacle
func_nav_avoidance_obstacle is a brush entity available in the 
Left 4 Dead series.
 class hierarchy
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CFuncNavObstruction defined in nav_entities.cpp
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It tells bots that nav areas in this volume are obstructing pathfinding but can be ignored for longer-term queries like computing flow distances and escape routes. Bots will go into the affected areas if their destination is in the obstructed area, or they must cross the obstructed area to reach their destination, but otherwise avoid it. Used sparingly to prevent bots from moving to odd-looking places.
Warning:AABB is used for LOS obscuring, so clipping or rotating brushes does not change a brush's size as one expects.Diagonal walls will always be ignored, as it will take the furthest out corners of the diagonal walls and create one huge box around all of it. That box will be your brush entity.
Uses in Official Maps
l4d_hospital03_sewers / 
c8m3_sewers
- 1 used on gas station to prevent bos going near it
l4d_airport02_offices / c11m2_offices
- 2 used inside the office on office dividers to prevent bots slowing down doing hard jumps over them
l4d_airport04_terminal
- 2 used in metal detectors to prevent bots triggering them
l4d_river01_docks / c7m1_docks
- 5 of them are used on the wooden structure near the sea by a path of rocks under the bridge.
- 1 used on a window where survivor bots have hard time jumping into
- 1 used on barrels in the brick factory to make bots go around
l4d_river02_barge / c7m2_barge
- 1 used on the first water body to make bots go around it
- 1 used on the ledge of the coal barge in the second water body to make bots not go near the ledge
- 1 used on the way from top conveyor belt to the bottom one to prevent survivor bots trying to follow you through there if unnecessary and hang
c13m2_southpinestream
- 2 used in an attempt to prevent bots going into fire but enabled trigger_hurt already has that effect
