Yeah, Kol's right... monsterclip.
But while we are on the topic of Sci's stopping and refusing to take another step......
This normally happens because the Sci senses danger. A monster lurks around the corner, or there is every chance the Sci is about to be killed.
Keep this in mind when building corridors next to each other, even though you can't see the monster, the Sci will know it is there.
You don't have to info_node the place. When called, Sci's don't need info_nodes, but sometimes it helps.