Home Alone viewers only just realising how Kevin actually got left in the first place

It’s so close to Christmas, and you just know the TV guide is going to be packed full of your favourite classics for a few days.

And there's a ton of classic Christmas films to choose from, like Love Actually, Elf, and The Polar Express, but there's one more that’s got to be watched before the big day - and that’s Home Alone.

The 1990 film that kick-started an eight-year-old Macaulay Culkin’s career as he takes on the role of Kevin McCallister, who is left home alone (get it?) after his massive family forgot him when rushing off to Paris for the Christmas holiday.

When Kevin wakes up alone, some not-so-cunning robbers take the opportunity to break into the house, where Kevin's quick wit allows him to protect his home.

The Home Alone movie is one that just brings the whole family together. It's got Christmas comedy, a stellar cast, and heartfelt moments.

However, some people are sticklers for details, and were left scratching their heads when wondering just how his family could have left him behind.

Home Alone is a must-watch at Christmas (20th Century Fox)

After figuring it out, fans of the rib-tickling movie flocked to social media in astonishment after only 'just realising' how chaos-causer Kevin first ends up alone in the first place.

Yes, that's right. Hundreds of viewers have leapt to X (formerly known as Twitter) in surprise after clocking the easily missed Easter egg, which explains exactly why Kevin didn't make it on holiday with his family.

In the first movie, Kevin's parents, siblings and other family members only realise mid-flight to Paris that they've left him behind.

But why does it take them so long?

They woke up late that day, with their alarm clocks having been reset by the electricity cutting out. They then had to rush out of the house, perform a quick headcount of the children - unfortunately, counting a neighbour and missing out Kevin - before rushing to catch their flight.

And in the second film, Home Alone: Lost in New York, Kevin also gets left behind again in the sprint to the gate at the airport, distracted by his camera.

But it's none of these incidents which are the main factor in Kevin not making it on the same flight, at least in the first movie.

Macaulay Culkin as Kevin McCallister in Home Alone (20th Century Fox)

Fair enough, Peter (John Heard) and Kate (Catherine O'Hara) McCallister do have five children, but you'd think they'd have realised if they'd forgotten to check one of them in.

Unless something had happened to Kevin's plane ticket, that is.

In the scene that takes place the night before the family's trip to the airport, milk is spilt over the family's passports and plane tickets after Kevin and his older brother, Buzz, get into a fight.

Of course, out of all the plane tickets which accidentally get used in the clean-up, whose should it be but Kevin?

After using a bunch of red napkins - Kevin's ticket concealed within - to wipe up the spillage, the plane ticket ends up discarded in the garbage, a plot point that was originally in the film before being removed in the final edit.

You can spot the ticket in the bin in a brief camera shot, which zooms in on the wet napkin mess.

However, not all viewers spotted the Easter egg until now.

Fans have since cracked the case (20th Century Fox)

One user said: "Okay this is crazy lol and I'm sure people have found this Easter egg many times, but for me I just realized that in Home Alone, Kevin's dad accidentally threw away Kevin's plane ticket. Which explains how they didn't realize anyone was missing handing out tickets at the gate."

Another wrote: "I just noticed on home alone when they was eating pizza and Kevin started the fight with his brother, the dad threw Kevin plane ticket away im slow never noticed that."

A third commented: "30+ years later I just noticed that the dad in Home Alone throwaway Kevin’s plane ticket?"

While a final said it 'adds context' to Kevin's final line in the sequel, which was: "It's a good thing that I have my own ticket, in case you guys try to ditch me."