Maybe you guys are familiar with the good old "Spectator switching" bug in tf2. If you shoot a projectile of any sort and then join spectators, your projectile will harm your former team. Well, it may be fixed in tf2, but it's still available in pvkii as you guys can see in the video below.
...But there's more. There's one more ff abuse specified for knights' team in trinket wars maps which have explosive kegs in it. I'll show the demonstration in the video below.
Note: This one only works with knights' grail.