Karbine: My (primitive) understanding is that most MMOs use estimation of positions. Your character will move on my screen based on the speed and direction you were moving at my last confirmed position for you. If you stop or run into a wall, it will be corrected on my screen at my next sync with the server that transmits your position to me. It's possible that if you see the tank up on the ledge, but the tank shows himself against the wall, you're in that hangtime. And your client says, "No, I can't see him," and doesn't allow the cast.
Again, this is just my very basic understanding, and could be entirely wrong. I'm sure the legions of experienced developers will be along in a few hours to correct any mistakes.
Ekko: Lag. It takes time for your action to go from you to the server, and then the server to the other players.