<playground player: Is it just me... or can it really be harder, sometimes, to beat a true patzer who's just making it up as he goes along, than it is to beat somebody who has a fair idea of what he's doing? I mean, some of these opponents who never even heard of "the book" can be pretty tough.>

It's possible. In contract bridge, for instance, an expert player can usually draw certain inferences from the opponents' bidding only when they are fellow experts. When they are weaker players who misuse the bidding system, the inferences are not as reliable.

In chess, there's another factor as well. Adrianne De Groot, author of "Thought and Choice in Chess" did an interesting experiement. He took a group of players of all strengths, and showed them a position something like this:

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Each player looked at the position, then was asked to reconstruct it on another board. There was a clear correlation: the stronger the player, the more accurate the reconstruction.

Then he asked them to look at a position like this:

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With the pieces arranged more or less at random, the Masters were no better than the amateurs at reconstructing the position.

The point was that Masters have a large store of "normal" formations in their head which they can sense and evaluate at a glance. Take them out of these "normal" positions, and it takes a while to readjust. The non-master, with fewer preconceptions, may be able to orient themselves just as quickly.

Now, the fact that they are Masters means they are still more likely than the opponent to figure out what's going on, and since such unusual formations usually have some weaknesses they may win faster as a result. But it's more work, which means accidents can happen.