When Royal Mail just can't get it right and struggle to deliver your parcels on time. Sometimes they will deliver your items early such as the very next day after you post it whilst at other times they can take twice as long as the estimated delivery time and you get worried about where the parcel is. They are kind of like a rollercoaster between good and bad in a way.
Person 1: When will my parcel arrive.
Person 2: Whenever Royal Mail delivers it.
Person 1: I've waited a week.
Person 2: I know Royal Fail can be like that sometimes.
When Royal Mail employees are so lazy that they expect to deliver parcels late for £10+ per hour otherwise they collectively don't bother turning up to work at all creating the Royal Fail.
Joe: "Yo, Dave, did you get round to buying those creps for your missus?"
Dave: "Nah, mate. Otherwise I'd be waiting over a month for the Royal Fail to deliver them, better to go with UPS"