Yes, contamination is very easy. You should always pour what you need out of the jar and then wipe down the inside of the cap and threading with a clean paper towel before closing again, and store in the fridge for maximum life. But even so, you mayu notice suddenly all your clones start failing. It really should come with a 'use within 4 months of opening' warning on the label!
I switched to dip-n-gro. My friend gave me the dregs of a little bottle of the stuff he's had for oh hell, easy 6 years... it brought my success ratio up over 95% the FIRST TRY and now I get 100%. It has a fungicide and is more stable chemically it seems.