Check baggage restrictions!
Budget airlines, for all of their greatness, love to stick it to you on weight or size restrictions to bags. I got caught one time at the bag check with a really hefty bill (over £100) because my bag that I had been using for my month of travel between the UK, France, and Germany was wayyy over the limits for Ryanair. So my options were to pay it or trash my bag and everything inside. Lucky day for Ryanair, I paid it. After that I try to be aware of such restrictions and the fees associated. Some airlines will charge you to check a bag while some won’t. Some airlines will make you prove your carry-on bag will fit in the stupid sizer (and they don’t care if you break your bag trying to get it out). Some airlines have weight limits on carry-on bags.
It seems like fine print, but it could cost you quite a lot of time, effort and money to get your bag from point A to point B.