Captain Anorak's Guide to Gaming
Oracles
Too often, scenarios rely on oracles. Call of Cthulhu campaigns published by Chaosium are particularly bad for this. An oracle is an NPC who knows effectively everything and is willing to tell the PCs some of this out of the goodness of his heart. Now that may sound like a stupid idea, but I've seen it happen far too often.

Here's an example from an old CoC campaign. In a certain forest in the Dreamlands there's a glade where a god sits. Dreamers can visit the god and ask questions of him. The scenario says that he knows everything and will discourse on any mythos topic.

Now, the PCs find out about this god when they are researching a specific question: they find a note in a book saying that this particular god would know this, and says how to find him. The idea seems to be that the PCs should go to consult him once on this one topic, and maybe ask a few other questions while there, but then go away and never return.

There are two problems with this:

1. It allows the PCs access to far too much knowledge. When they turn up, they can ask the gods all sorts of questions which basically boil down to asking for the plot of the scenario. They can ask, 'What is this thing we're investigating? What does it do? Where did come from? How can we stop it? What will its effect on mankind be if it is left unchecked?' It would be bad for the players to find out this information at this stage, because most of the campaign revolves around searching for answers to these questions. So the only answer left for the GM is to say that the god chooses not to answer that question. This gets really annoying after a few times.

2. The PCs can go back to the god again and again and ask questions about anything new they find out about. Such a useful resource as an all-knowing god who answers questions will not be used once by the PCs and then forgotten. He will be a constant reference source. So each time they uncover a few new clues, the PCs trundle off to the god and ask him about them all. Then again the GM has to say 'The god's not answering your questions' because, once again, it would cut the camapig extremely short if they found out about this sort of thing.