I once booked a flight to meet my then-fiancee in Florida on vacation. Work travel came up unexpectedly, and I booked my work travel from ORD > SFO > TPA.

Before I made that booking, I called the airline specifically to ask them if skipping the ORD > TPA leg of my personal travel was going to cause me problems. The agent confirmed, twice that it would not. This was a lie.

Buried in the booking terms is language meant to discourage gaming the system by booking travel where you skip certain legs. So if you skip a leg of your booking, the whole thing is invalidated. It's not suuuuper clear, I had to read it a few times, but I guess it kinda said that.

Anyways - my itinerary was invalidated by skipping the first flight, and I got lucky enough that someone canceled at the last minute and I could buy my own seat back on the now-full flight for 4x the original ticket price I paid (which was not refunded!).

I followed up to try and get to the bottom of it, but they were insistent they had no record of my call prior, and just fell back on "It's in the terms, and I do not know why you were told wrong information". Very painful lesson to learn.

I try and make a habit of recording phone conversations with agents now, if legal in where I'm physically located at the time.