From inside info on combo meals to being refused service for wearing a tracksuit, here are some fast food facts that might surprise you.

Combo meals can make you spend more

Combo 'meal deals' are usually offered to customers, even when they are ordering a single burger. Lisa, who worked in an American-style fast food establishment, remembers: "They would always want us to sell combos and try to upgrade drinks and fries to larger sizes".

Although it seems a good deal, only ordering the items you want can be cheaper than paying for a combo with extra items you don't want. But you still have your fries and drink, right? Researchers at the University of Virginia found that 15% of people would not have ordered fries with their meal if a combo deal didn't exist. By comparison, there are sandwich deals around that bag you a drink, sandwich and snack for under £3.

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'Healthy' and 'kids' options might not be that healthy

Sandwiches, salads and kid's meals are not always the healthiest options, according to a 2009 Which? report. Researchers found a pre-packed sandwich containing the equivalent of five teaspoons of sugar and one restaurant's six-inch meat-filled baguette packed 4.7g of salt — three-quarters of an adult's recommended daily allowance.

Since then, restaurants have worked to reduce salt levels — but check before you buy. We found one chicken and bacon salad containing more calories and fat than a box of six fried chicken nuggets. Maria, who worked in a pizza restaurant, wasn't surprised: "We were told there were more calories in a kid's cheese and tomato pizza than there was in a burger meal with full-fat Coke".

They can refuse to serve you for unusual reasons

Although our fast food workers never had to refuse to serve a customer, it does happen — and for bizarre reasons.

A woman who drove a horse and cart through a McDonald's drive-through in Derbyshire in May 2011 was refused service on health and safety grounds.

A Burger King in Hove refused to serve lunch to a group of teenagers in 2008, because it was before 3pm.

And in January 2011 a cashier at a McDonald's near Preston, Lancashire refused to serve a teenager because he was wearing a tracksuit. He returned, 15-minutes later, wearing a shirt and tie and was served.

Fast food staff often work longer hours than you think

24-hour restaurants offer fast food round the clock, and it can mean employees have to work longer hours. Sarah, who worked in a burger restaurant, remembers one night when a drunken customer was ill in the toilets, towards the end of her shift.

"I was given a mop and bucket and told not to leave until it was cleared up, leaving me still there at 1am". Lisa, who worked while studying at college, agrees: "They expected me to stay until 2.30-3am on a school night just because someone else was late for work."

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Employees have their limits

If you think only customer tempers flare up at fast food restaurants you'd be wrong. Faulty equipment, impatient customers and long working hours can put extra pressure on workers. Lisa recalled a combination of a faulty drive-through speaker and a surge in orders almost pushed her over the edge one night.

"Occasionally I would lose my temper and have to sit out back to cool off", she said. Others were less diplomatic. Early in 2011 a KFC worker in Australia was suspended after yelling abuse at a customer and had to be restrained by colleagues. It was captured on video camera by the customer and made headlines.

Lapses in hygiene do happen

All the big fast food companies state they have strict hygiene policies in their restaurants. However, in 2008, at one busy Leicester Square fast food restaurant, inspectors found flies, a mouse and a cockroach nibbling on a chip. They had been taking 1,600 orders a day. The restaurant was closed, completely refurbished and new management instated before reopening.

Children's parties can be hard work

Many fast food restaurants offer children's birthday parties, with food, balloons, games and a birthday cake. But spare a thought for the staff running your party.

Maria remembers: "The parents would dump twenty-five 8-year olds on you and do nothing to help and then wouldn't even tip". With their colourful décor and easy to eat foods, fast food restaurants appeal to children, but Lisa adds: "Some children really misbehaved, and we had to deal with it".

Names have been changed.

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