Your homeland security make us fill in an online form to be accepted for entry before we leave our home country.

Electronic System for Travel Authorisation(ESTA) At the end on the acceptance page you get a massively long number. It says take a note of your number but you DON'T need to print anything out.
Well
DCL demanded print outs last September as we were actually entering the US this way. Very few folk had them printed out for the reasons above. So much time and annoyance at Barcelona terminal as folk either input their numbers and then had to print their acceptance out. One friend just couldn't get accepted. Took 2 hours. She was told on no uncertain terms that if she didn't get a print out of her form she would be offloaded mat Madeira, the last European port, and make her own way home! Fortunately then for some reason I had printed mine off, and we did receive notification from DCL prior to the cruise that we would need them. Most folk didn't think they did though.
Fast forward to this cruise. We were already in the US, accepted by Immigration, so I had decided to leave my form at home. No letter from DCL. Bad idea! We were checking in at the terminal and the guy said 'where are the rest of your forms?' 'They are all there' I said. 'Your Esta form' he said. 'We don't need them' we said!!! OK! We had to go off to the furthest desk and input our numbers. That desk wasn't manned all the time, the person went walkabouts, and the printer was behind the screen! What a flipping pain it was. My anxiety levels were rising by the minute. Nobody said sorry or anything. One supervisor did come over and said that DCL had sent letters out to us. Mmmmmmmmm. When we eventually got back up to check in(and wait again) the lady there said they had only found out that morning that LA Immigration would want to see them!!

For goodness sake, we were going from the US to the US, having already been given clearance. Grrrrrr!
You did ask.