Google is big into their easter eggs in search, so spotting an unintentional one might be a bit of a challenge. One searcher spotted one and posted it in the Google Web Search Help forums.
A search using Google Voice search for [What is an equation] returns the definition, but when Google reads the last part of the definition off, it reads the the equal sign as a form of a smiley face.
Here is a video of it in action:
Forum discussion at Google Web Search Help.