When spiders throw web on you, you sometimes have to move away one tile (or more) to shoot them (but you can't because you are webbed) - so you can't shoot them with any weapon when you see them. But they can (spit poison and web). Don't know if this happens with other enemies, but spiders are most annoying.
Attached screenshot as example - i can't shoot targeted spider and can't move either, but spider can attack me from distance.
