This means that even the untrained may get the second snippet of important information on trolls (DC 15: they regenerate all wounds beyond fire and acid!), but only the trained will remember the third bit of information (DC 20 if you stand and fight them hand-to-hand, they'll grab you in both hands and rip you apart, so it's better to use hit-and-run tactics against them).
Im not sure I agree with this, I mean Im not trying to tell you how to run your game, just a friendly dispute about interpretation. See I think the fire and acid thing is only common if trolls are common. Personally Ive never fought a troll in 3e, but of course while *I* knew the vulnerabilities (being a well-read MM user) I wouldnt use them against trolls unless I was trained in the knowledge appropriate (we used Knowledge (Monsters)).
It seems if someone described a troll to me, however, I could imagine that their obvious offensive would be their over-long arms, almost gorilla-like and their sharp claws. Nothing about a physical description, drawing or artist's rendition, or seeing one in the distance suggests they are difficult to kill except with acid and fire. I guess what I'm getting at is the untrained knowledge checks should not be considered to be free "gather information" checks about the monsters, but rather a character's extrapolation of abilities based on what he knows about it.
Of course, it won't probably ever be a big deal (in a 4-person party, most low-level monsters' vulnerabilities are fairly well-known), but I think itd be funny if a long-time player who knew the vulnerabilities had to wait for the newbie to roll the dice, me to tell the newbie, and the newbie to tell him that you kill trolls with fire and acid

Technik