In Wintergrasp,
You can take one of the many ways onto the fortress right before the battle. If you are where a cannon spawns. For about half a second, Its Friendly to you.
If you Spam-Right Click. you will be able to take over a cannon. all other cannons will be Hostile. and you can destroy them with yours.
These cannons will NOT harm walls or towers.
But its fun to have the Opposing Faction Kill the cannon you are in.
(first post, don't find many useful exploits on my own, also searched and nothing on this topic was posted before.)