Mobs can't walk through thrown tridents, which can be observed easily with trident wielding drowneds building safe zones around you.

At first I thought it was a pathfinding issue, but they're really colliding. They can't walk through.

This can be exploited to create barriers that mobs can't detect, by creating a small gap with two tridents you can make mobs get stuck without trying to go around them.

Notes: Unsure whether this is a beta only bug or if it's in the current release, never noticed it before.