Travel hacking could be anything, from booking error fares, breaking routing rules with hidden city ticketing, or just working to the limits of the rules. Booking error fares and breaking rules can be harmful to your mileage balance. Airlines have closed accounts, cancelled tickets, and taken miles from people who break the terms and conditions. Usually it requires a pattern to emerge, but only one infraction could cost you all your miles. They can also bill you the extra cost you avoided when you do hidden city ticketing or just void your ticket.
Lonely CRJ-700 Taxiing at CLT
I'm not a travel hacker. I just enjoy taking advantage of poorly thought out marketing decisions. I stay within the rules at all times. I do work somewhat at the bounds of the rules, but make sure I am always on the good side of the line. There is no hack to my process, just a good bit of leg work researching deals and finding flights. There is plenty of value to be had staying within the rules, so don't risk your gains for a few extra points.