When the European Union's head office starts putting out statements saying "there should be no doubt that Europe will continue to assist Greece in whatever way is necessary," it's a clear sign something bad is happening.

This week, Europe's markets have been hammering Greek stocks and bonds like it's 2009 all over again.

That's prompting fears that the euro zone's weakest link could become the catalyst for another deep, dark crisis for the European currency bloc.

"Stocks, rates, oil: the great fear returns to the markets," headlined the French daily Le Monde on Friday as the jitters spread beyond Athens to shake indexes across Europe and beyond.

The prospect of political instability in Greece is just one of the factors spreading panic to the markets.