Users stung by excessive data charges due to IOS 9 Wi-Fi Assist

In a good example of 'The greatest harm is often caused by the best intentions' rule, a feature which is enabled by default in IOS9 is reported to have cost users significantly when it switches the data network option from Wi-Fi to cellular data when a Wi-Fi signal is weak. By changing the network without the user knowing it is possible to use gigabytes of data without realising which can result in hefty ISP charges.