Ryanair passengers could find themselves facing a fine through no fault of their own.
People flying with the budget airline are charged £55 if they don't check in online in the right way, which means online and before they get to the airport.
If they fail to do so and their flight departs in less than two hours, then Ryanair charges an extra £55 fee, or £30 for flights from Spain.
If you have checked in but you have not printed your boarding pass, you will be charged for the boarding pass reissue fee, which is £20 per passenger.
The scheme can be frustrating enough for those who get caught out by it, but particularly those who fall foul today.
The airline has warned that its website and app will be down for 11 hours between today and tomorrow - meaning there will be no way to check-in online.
If you have a flight due to depart from 6pm this evening until Wednesday, November 9 at 5am should check-in before the IT work begins.
Ryanair said in a statement on social media: "Due to planned maintenance, Ryanair website/app will be temporarily down from 6pm, Tuesday 8 to 5am, Wednesday 9.
"Customers who are due to fly during this 11-hour period should check-in online before 6pm on Tuesday, 8 November."
To view all of the terms and conditions when it comes to Ryanair check-in, click here.
Ryanair online check-in opens 60 days before the scheduled departure time if you have purchased an allocated seat.
Customers can be allocated a seat free of charge if they check-in online between 24 and two hours before each booked flight.
Once you have checked in online you can print copies of your boarding pass, or download them to your mobile phone, up to two hours before the scheduled departure time.
Your boarding pass must be printed on a single A4 page or downloaded through the Ryanair app. If not you could face a fine.
Ryanair does operate airport check-in but charges apply.
It's also important to note some airports do not accept Ryanair's mobile boarding passes. These are:
- Kefalonia
- Turkey
- Morocco
- Israel
- Lebanon
Customers traveling from these airports must check in online and print out a paper boarding pass for their flights.
It's also worth keeping an eye on hand luggage allowance.
Ryanair lets passengers bring one small personal bag onboard for free, such as a handbag, backpack or laptop bag, measuring no more than 40x20x25cm.
If you want to bring a second bag, you can pay to add 'Priority & 2 Cabin bags' to your booking.
This allows you to bring your small personal bag, as well as a 10kg wheelie bag measuring no more than 55x40x20cm. Alternatively you can also pay to add a 10kg check-in bag option to your booking.
