The most common sources of errors in official precipitation measurement come from equipment malfunction. If the tipping bucket rain gauge breaks down, then rainfall will not be measured accurately.
For weather enthusiasts who measure their own rain, common sources of error are a bad location of the rain gauge. If the gauge is situated close to buildings or under some other kind of shelter, then it will under-report rain.
11:20 EST Since the the royals stepped off the plane on Wednesday afternoon, Sydney has turned on the great weather.