Necro or not, I can see that my previous post above is no longer quite true. I can think of a number of team mate abilities that can be greifed - Newcastle ult, Rampart walls, Caustic barrels, Catalyst tactical. Most of these have some sort of reasoning behind them, but I don't understand why you can destroy your team mates Catalyst traps. That last one, I suspect is not by design, and for all I know it may have already been fixed. There's probably several more, that's just what came right to mind.
@BigMacAttack8u Absolutely not. You need to be able to activate your Caustic team mates' barrels, and you need to be able to destroy them when your team mate has cucked you with their placement. I have a Caustic friend who has gotten me killed countless times by blocking doors I needed to use. I imagine that sometimes your random might grief you by destroying your barrels, but it's necessary to be able to do so IMO.
