I've occasionally noticed NPCs getting stuck on various blocks that they could trivially hop on/over, but it doesn't seem to occur to them to do so.
Latest example:

That's an Immersive Engineering sloped conveyor belt keeping the worker from getting to his upkeep point a whole 10 blocks away. The source of the confusion here is pretty clear: the actual block is below the space the top part occupies, so I'm pretty sure the NPC thinks it's trying to walk into an empty space and hitting a collision box it has no notion for. I don't know how easily that can be addressed without going the route of special handling for specific mod blocks.

Something a little less reasonable: farmers insist on believing they can walk through wildberry bushes from Rustic. Given that there's an actual bush right there in the voxel in question, I don't know why they don't pathfind around or over it.