The more we rely on contact-less payment systems the more problems will arise.
Knowing exactly what your balance should be will be the big problem as like credit cards etc until you look at a statement people often forget what they used it for or in this case when.
Cards can be cloned from a metre or so away.

http://www.dailymail.co.uk/news/article-6571697/Sydney-transport-bosses-REFUSE-refund-incorrect-Opal-fares.html

Sydney transport bosses will REFUSE to refund incorrect Opal fares - even if customers have been overcharged

Sydney transport will refuse to reimburse incorrect Opal card fares after customers reach an ‘error limit’ even if they are entitled to a refund.

One woman, who spends $75 on public transport a week, was told she'd 'reached her customer error limit' after she reported three mistakes in a year.

Transport for New South Wales has been set on the amount of refunds allowed, leaving commuters out of pocket.

Customers who believe they've been overcharge by the contactless payment scheme can ask for a refund but only within 90 days, 9News reported.

A Transport for NSW spokeswoman confirmed there was no limit on adjustments to Opal Card balances made 'as a result of equipment or outages on the transport network'.

'However, there are limits to the number of adjustments that are allowed as a result of customer error, for example, when a customer does not tap off correctly.

'This encourages customers to remember to tap their Opal cards on and off correctly to avoid receiving a default fare or risking a fine of up to $200 for fare evasion.'

The spokeswoman said the limit to error adjustments 'has been determined through extensive analysis of customer needs and revenue protection.'

Customers can view their trips and costs via their online Opal card account summarised in a statement.

Transport NSW will also begin to direct passengers online for Opal issues and timetable queries instead of calling their customer service helpline.

'We're going digital. Some of our phone services are now only available for customers with a disability, or who need extra assistance,' the spokeswoman said.