For example, in my teens I found a hack in the eBay search algorithm. The algorithm was very simple; it simply scanned the titles. So if I input '32GB Apple iPod Black' it would return everything with those keywords. If, however, I typed in 'iPod' it would return millions of results and be unusable. Not everybody realized this, however, so there were plenty of items just titled "iPod."
I ended up generating a 2-page long search that stared with iPod and negated all non-relevant terms (i.e. 'ipod -headphones -case -protector -battery' etc.). After entering in that term I would be given a page of everything only titled 'iPod,' and could pick them up for $50-100 and flip them the next day for $250-300. That was a good living for a 15-year-old kid.
When my mom began to probe after seeing iPods arriving at the house day after day, she started to question me, and we had a long philosophical discussion about whether or not what I was doing was "right." It was legal, of course, but was it right? - this was the much more important question in the eyes of my mom. She felt it was unethical and not providing value - I disagreed, and she allowed me to continue and not go against my conscience.
After a few interesting twists and turns the eBay hack turned into a multi-million dollar, off-eBay Broadway ticket brokerage firm. (We scalped broadway tickets at scale.) I provided no real value other than buying tickets at face value a long time in advance, and marking the prices up 3-4x last minute. I justified it by saying wall-street traders don't provide much value either. I let people wait to buy tickets, and let Capitalism decide who would get the tickets instead of who got word of the shows first or who cared the most. I appeased my conscience by convincing myself that I took the risk away from the venues, who could have waited and charged more, and profited thereby, but either didn't know it or were bound into agreements with artists/shows/etc. I still don't know how truthful that was, but eventually I left to try and start something that will actually benefit people, not just redistribute things and make money.
I wonder now if I was on a slippery slope and got off at just the right moment. I've seen a lot of my ticket broker/scalper friends end up trying to swindle casinos, get caught up in credit card fraud, etc; it becomes easy to feel smarter-than-thou when you're making bunches of money off of others' perceived negligence.
But at the same time I'm not the kind of successful where I can look back and say, "I was dumb and broke the rules and made some mistakes, but look, I made it now!" It's tempting to slip back into a world where you're not benefiting anybody and profiting thereby. So in that sense it's hard to separate my motives then from that of the author. Making money while helping people is really, really, really hard.
The moral of the story: It's difficult for me, at times, to discern when a "cool hack" becomes "criminal theft."