There are a few classes of mistakes that people often make. The first has already been mentioned a few times: Not being specific enough. This actually has two incarnations, however. You can not be specific enough in your search terms, or you can not be specific enough in your search engine.
For example, if someone wants a map of Portland, OR, they may want to go to google maps if they're trying to find a street address, or to google images, if they're looking for a nice image of Portland marked on a map of OR. If you're looking for a biography of George Washington, you may want to do an Amazon search to find a book, or a Wikipedia search to find information.
I think a solid covering of the second class of mistakes would help you expand your discussion significantly. This also flows nicely into another facet of using the search engine, which is looking at the url to see where it's going to take you.
Additionally, people often end up searching for a specific answer to a question, when a search for a service would be a better idea. This is the opposite of the first problem of not specific enough search results, and is related to the second problem of not specific enough search engines. Instead of googling "John Doe's phone number", googling "Jane Doe's phone number" then "Sally Doe's phone number", it would be a better idea to google "find phone numbers" then stick to the white pages search.