Bob, an ogre, and Tony went out to lunch yesterday.
The problem here is that the first (Bob) and second (an ogre) items in the list can also — grammatically correctly — be read as an apposition, wherein 'an ogre' is presenting additional information about Bob rather than being a list item in its own right.
So... setting aside the fact that what was actually meant should be fairly clear from the context, what the reader gets out of that sentence is going to depend greatly on whether he or she knows Bob.
Now, one could argue that the confusion is easily dismissed (provided, that is, that Bob and the ogre are distinct items) by simply removing the serial comma —
Bob, an ogre and Tony went out to lunch yesterday.
But don't you dare! Far better to rephrase as either
Bob, Tony, and an ogre went out to lunch yesterday.
or, alternatively, as
Tony and Bob, an ogre, went out to lunch yesterday.
depending on what, exactly, you're trying to say.
Don't let this occasional hiccup sour you on the serial comma; it's a relatively-rare and easily-correctable situation. And let's keep in mind that no matter what punctuation rules you choose to follow, the responsibility for your writing saying what you had intended lies with you. Don't blame the comma.
SO — use the serial comma at every chance you get, but use it wisely. No one appreciates being called an ogre, after all. Even accidentally.