My former boss would always say, "You can screw-up, but you can never "f-up."
Screwing-up is making a mistake like dropping off a FedEx package 5-minutes after pick-up is scheduled for the night. F-ing-up is lying to your boss to say you made it to the dropbox by the deadline and have no idea why FedEx didn't pick up your specific package. Then creating an elaborate story how, after calling FedEx ten times to demand the whereabouts of your package, you found out the FedEx truck broke down during a rainstorm and packages would arrive the next day.
I actually observed the above story play out and it wasn't pretty. This guy F'd-up majorly. FedEx rarely makes mistakes; they track everything and give you immediate answers. The lie unravelled quickly and made the assistant look like a schmuck never to be trusted. It was a matter of time before he was out.
We all make mistakes. It's better just to fess up in the beginning. When you admit fault, you engender trust and pave the way for a long relationship.
Years ago, I went to a new hairdresser who came highly recommended. I made an appointment for 9:30 A.M. on a Saturday. I got there at 9:20 and waited and waited for my new hair person to arrive. Around 9:40 the receptionist said the hairdresser was on the phone and wanted to talk to me. Keep in mind this was the first time I had ever spoken to her. I picked up the phone and she immediately apologized for not being there. She said she normally doesn't make appointments earlier than 10 A.M. and that she just screwed-up. She'd be there by 9:50 and was really sorry for the inconvenience. She could have very easily blamed traffic, but it just wasn't true.
That small experience with her told me that this woman would never lie to me. I've been going to her for eight years now and she personifies integrity.
It is hard to admit fault, but really, it will save you pain and anguish in the long run. So, when you screw-up, just fess-up!