3. Investing in AOL, 2005

If someone told you to invest in AOL today, you would probably refrain from taking anything they said seriously from that moment on. However, back in '05, Google saw fit to plunge $1 billion into America Online for a 5% stake in the company. Suffice to say, Google didn't get a good return on its investment. In 2009, AOL's revenue fell by 23%, and Google sold its stake for $283 million. Ouch.