Incidents in which an airplane vanishes mysteriously are rare, but they have happened before. Before Malaysian Airlines Flight 370, there were these 10 mysterious plane vanishings, many of which remain unsolved.
While this list shows that the Malaysia Airlines mystery is not a once-in-a-lifetime event, it's notable that most of these are from before 1990, despite the incredible number of flights in the modern era. Our ability to track aircraft and to keep them safe in the air has improved immensely in the last twenty years. That's what makes the missing Malaysia Airlines plane all the more mysterious.