When something unexpected happens that affects one economy (or part of an economy) more than the rest. This can create big problems for policymakers if they are trying to set a macroeconomic policy that works for both the area affected by the shock and the unaffected area. For instance, some economic areas may be oil exporters and thus highly dependent on the price of oil, but other areas are not. If the oil price plunges, the oil-dependent area would benefit from policies designed to boost demand that might be unsuited to the needs of the rest of the economy. This may be a constant problem for those responsible for setting the interest rate for the Euro given the big differences--and different potential exposures to shocks--among the economies within the Euro zone.