While we are talking about 'throwing away' flight segments, does anybody have experiences to share that involve not showing up for a segment (not the last), walking that segment and then continuing?

I tried this once. Had a non refundable non changable YUL-FRA-YUL ticket on AC and needed to change the departure date, which wasn't possible. Found a really cheap new ticket to FRA on AF. However, I wasn't able to get a reasonably soon return date for the AF ticket. So, since I stayed very close to the airport, I thought I'd give it a try to use the return segment of my AC ticket. I removed the first coupon from my ticket, put on my best tourist attire and approached the check-in agent with a straight face, pretending that I had no clue what was happening when the system complained about the YUL-FRA no show. Fortunately I didn't have to lie, since she didn't ask a single specific question - so I was able to get on the flight no problem. But I was very nervous at the check-in!

Me thinks I was very lucky. But is it unethical? Should I feel guilty? Thoughts?