There are tons of anecdotes i remember from AI classes about neural nets learning something other than what you expected them to learn from your training set.<p>For example, one story involved training a classifier to recognize an overhead image with tanks vs without.  It turns out it ended up learning which days were sunny and which were overcast.<p>This sort of thing happens when training people from examples too:  From the mundane cases in school, to the AA587 crash in Queens NYC.