"Assume Bad Faith" (yes, the link is intentional, go on, click it), as applied to vandalism and other destructive behavior, really is just sort of sarcastic. What it means to me is that we have to ask, with some forms of vandalism at least, whether there is more going on then just malicious behavior.
I have seen this at the Runescape Wiki, and finally had to find the weight of various pieces of Hunter gear (don't ask), and provide screenshots from within the game to clear the matter up. It seems that RS Wiki editors assumed that the weight of the objects is zero, and while a cursory fact-check confirms this, a deeper look revealed that there was a rounding-down error involved. IP contributors and other users were so exasperated with their entirely more accurate data being removed, that they became hostile and destructive out of sheer frustration. This is the sort of situation, perhaps easily avoided, that "Assume Bad Faith" hopes to address.
There is a page by a curiously similar name over at Wikipedia: Assume bad faith