When a customer pulled up to a Starbucks coffee shop in St. Petersburg, Florida, she was surprised to be told that her coffee had already been paid for by the person in front of her. 

The woman had essentially just been given a free coffee, but instead of just taking the gift, she decided to return the favour and 'pay it forward', offering to pay for the drink of the customer behind her. 

The 'pay it forward' idea has become increasingly popular in the US, but Starbucks employees did not expect the chain reaction that this particular one caused.

The first 'pay it forward' offer occurred at 7am, and when the chain remained unbroken by 1.30pm, the Starbucks baristas kept a tally of all the drinks that had been 'paid forward'.

People ordered a drink at the speaker and when they pulled through to the next window, the barista, Vu Nguyen, 29, would inform them their drinks had already been paid for by the person in front of them.

He would then ask, 'Would they like to return the favour?, according to The Tampa Bay Times. 

By 6pm, when the chain eventually ended, over 400 customers had essentially been given free coffees, but had all decided to return the favour and keep paying the random act of kindness forward.

"It's nice just to do a random act of kindness for someone you don't know," said Tim Burnside, a customer at the Starbucks.