If a player is in spectator mode, they can physically prevent any projectile from travelling further (stop it completely) and also prevent players from placing blocks where that spectator is. I have tested this in both Vanilla Minecraft and on my Spigot server and have determined that this only happens with Spigot.

To reproduce in both environments, create a 3x3 wall with a hollowed out center. Have the spectating player go in the hole in the wall. Try to place a solid block in the hole. You will not be able to place the block when using Spigot. Then, try to throw an ender pearl through the hole and shoot an arrow through it. Both will be blocked by the spectator.