Current batches of MacBook have reported high incidences of intermittent shut down. Although, initial remedies in the form of logicboard replacements seemed to have cured the issue, Apple has localised the issue to stem from a heatsink defect rather than a logicboard fault.
If you have a MacBook that shuts down by itself without warning or one that starts up only to shutdown or restart again then the best move is to quickly send your MacBook to your nearest Apple Service Provider for a remedy.
At time of reporting, there has not been an official recall on this issue but Apple appears to be making positive efforts to remedy the situation.