Simplest example I cited already: it becomes obvious that company X will default in short order, but it's a member of an index and the market will "buy" it anyway: I can short it (by borrowing from the passives and selling it to them next time they buy the index) and never have to cover since it's bankrupt. That's a simple example of a single-player active skewering the passives.

You can try to figure out what this means in terms of tracking error but it's kinda irrelevant to my point.