As spiders wander around caves with no players to target they occasionally clime walls. It seems very often when a spider that is not targeting a player climbs a wall to the ceiling they get stuck. After getting stuck the spider cannot free itself from the climbing motion. They are continuously moving upward into the ceiling. The spiders that are stuck are not able to chase or jump at players, even if the spider is in a dark area or the player attacks the spider. If the player walks into the spider they will take damage proving the spider is hostile to that player but still cannot chase the player. If the ceiling the spider is stuck on is mined away the spider will climb higher until it reaches a new ceiling or until it reaches the surface where it seems to be freed from the bug. A player can also push or hit a spider away from the wall that forms the ceiling corner and the spider will be freed. At this point it attacks the player normally. It seems once a spider gets stuck it cannot free itself at all. Many of the attached screenshots were taken in completely dark caves using night vision potions to see. This can be tested by spawning a spider in creative mode such that you are not a target for the spider watching it wander until it climbs into a ceiling corner and seems to stay there for an abnormal amount of time. Than use the /gamemode command to change to survival or adventure and the spider will not move to attack you. You can than observe everything I have described here.