Bought a large jar of black olives and used half of them.  Why then, a week after using half, did I go to the jar and find a few mouldy clouds which made me recoil and lob jar into the bin.  The vinegar was more than covering the olives, I did not drain them.

Is it possible I contaminated the liquid?  Or should I have stuck to olives under oil instead?  I do remember thinking the large jar was quite cheap.