As the last decade came to a close, an unfortunate German woman went on her summer holidays to France. All was well until she decided to download an episode of Lost on her phone. (Remember Lost? What a disappointing ending.)

So she downloaded the programme and thought no more of it until she came home to find a phone bill for €46,000 waiting for her. This is probably the worst example of bill shock we have come across (apart from the cost of our bank bailout, that is) but it is by no means the only one.

Readers frequently contact us to complain about ridiculous charges attached to their mobile phone accounts once they go above an agreed data limit with their provider. And it only very rarely has anything to do with roaming charges.

A person might sign a €30-a-month contract and get a bill for more than €100 because they have breached agreed data limits.

Generally speaking, companies don’t go out of their way to alert people that they are coming close to their limit, because they know this is where the big bucks are to be found.

That is why many people sign up for unlimited plans. When you hear the word "unlimited", you probably think of something without, you know, limits. And if you do think that, it is not your fault. The Oxford English Dictionary defines the word as "not limited or restricted in terms of number, quantity, or extent". So that's pretty clear, right?

Unless you’re dealing with a mobile phone operator, in which case the word means nothing of the kind. In recent years, most Irish mobile phone providers have been advertising “unlimited” plans. The idea is very seductive: you pay an agreed sum (and it is usually a premium) for the peace of mind of knowing that you will never again have to deal with the horror of bill shock after you use large amounts of data.

The limits of ‘unlimited’

The problem is, “unlimited” means different things depending on which network you are on. None is truly unlimited. Depending on your provider, “unlimited data” might mean as little as 5GB a month, and “unlimited calls” could mean as little as 100 minutes a day.

This is because of fair usage policies, which rarely seem very fair for the end user. It is what various networks define as a “maximum acceptable” level of usage, and once you go above that you start being hit with charges.

Mobile operators put fair usage policies in place to protect themselves from excessive users, or people who are using the network for business.

But, confusingly, every operator has a different definition of "unlimited". For calls and texts, they range from 3,000 minutes or texts per month on Three and Lycamobile plans, up to a whopping 45,000 minutes or texts per month on Vodafone. The chances of you breaching any of these limits are quite slim, although 3,000 minutes only works out at 100 minutes or texts per day. But it can all get very confusing.

In recent months we have mentioned an Irish start-up by the name of KillBiller. The company has developed an app that finds out the best available mobile price plan for you without you having to put in any of the legwork.

You just download the app and it processes the details of the calls, messages and data you use to establish how much you would have spent on each plan from every network.

In recent weeks the people at KillBiller have put their minds to working out what “unlimited” actually means in terms of calls, texts and data. They have also tried to work out if we really need “unlimited” plans at all.

Shane Lynn of KillBiller reminds us that unlimited plans are more expensive than those that have limits, "and in reality many people don't use anything close to the generous allowances of cheaper plans". His company's research shows that typical users use far less than these limits. Average usage is 207 minutes per month, and sending only 123 texts.

Analysis of data usage tells another story, however. There has been a huge increase in the amount of data we use as we more fully embrace the Internet of Things (and upload pictures to Facebook).

According to the KillBiller data, the average traffic per smartphone in the first three months of this year was 1.6GB, and it is rising steadily.

As that is an average, it means that a whole lot of people are using a lot more data than the average. It is “remarkably easy to cross a 5GB limit when you are not connected to wifi and using video- or music-streaming services”, warns Lynn.

For example, 10 hours of video watched via the RTÉ Player in a month will take up about 5GB on its own. If you watch just one hour of HD video via Netflix on your phone, you will use 1GB; and if you upload 500 pictures to Facebook, that is another 1GB gone.

Another 1GB disappears if you spend three hours in a month on Skype: that is less than 20 minutes a day. If you download 150 songs from iTunes, you will use another 1GB. Streaming services can also eat into your allowance unless you are in a wifi zone.

This is why “knowing your operator’s data limit and fair use policy becomes very important”, says Lynn. There are six mobile networks in Ireland, each of which offers an unlimited option, although some have dropped the word “unlimited” in recent times.

"There's some great deals out there if you are not getting a phone with your plan," according to Lynn. Tesco has a €25-per-month deal and a 30-day sim-only plan for €30 per month. Three's Unlimited Flex Max can be had for €55 a month, with 15GB and a phone included.

“Bearing in mind that the average user only makes 207 minutes of calls per month, for many people the key differentiator between plans is data use,” says Lynn. “Before committing to a contract, try to understand your usage patterns and your requirements, and don’t pay for services that won’t be used on a monthly basis.”