The Problem with Wikipedia. The same goes for books like The Oxford Companion to the Mind, ISBN: 0198662246; I can't actually find anything in the damn book. I scan through to locate an article but on the way there I just can't help reading all those other shiny words.
On balance, I don't actually see this as a disadvantage of physical books. It allows a sort of 'side-ways discovery'. I suppose the directly analogous process on Wikipedia would be going through alphabetically, which is not what the cartoon was about. However, randomly following tangential keywords does seem to capture some of the chance aspect.
Another non-replicated property of books I like is the ability to search by image. For example: I was looking in Managing Gigabytes, ISBN: 1558605703, for a discussion of Hierarchical Bit Vectors. However, I actually didn't know that name at the time; I only vaguely remembered that it was nearby a diagram of the process. I knew what this roughly looked like, so I could quite quickly flip through the book, find the image, and then the nearby section. In principle, you could do this online if you had a PDF, yet I don't think you can scroll quite so easily as you can flip pages.