When I was working on my private, a CFI told me this story:
A young instructor is working hard and trying to build time. He wants a part 135 job really bad, but doesn't have the multi time. So one day he starts to pencil whip time into his logbook, using a Cessna 310' s tail number he sees at the airport. He gets the 135 job and everything seems fine. Eventually, he needs an ATP and goes up for the checkride. During the oral, the DE is spending a lot of time going over his logbook. Finally the DE asks " I see you have a lot of time in twin cessna 123XX. " The pilot responds in the affirmative. The DE looks at him and says "That's impossible, 123XX is MY airplane." Needless to say, the pilot's career ended that day.
Even if it doesn't seem wrong or unethical for some to pad their logbook, things like this tend to catch up with people. He'll always have to look over his shoulder.