Not so much a scam but more of muddying up the details. Read the fine print, talk to the customer service teams about the interest on promotions, etc. Basically - don't stack multiple promotions, as the one that is expiring first will get all of your payments towards it until it is paid off. If I remember right, it went something like...
Purchase A) $1000 purchase on 12 months no interest in Jan.
Purchase B) $500 purchase on 6 months no interest 3 months later, in Apr. (9mo left on first promo)
Purchase C) $200 on 6 months no intestest 3 months later, July. (5mo left on first promo)
Any and all payments go towards the No Interest Promotion that is expiring the soonest. So if you were making payments towards the Purchase A) $1k but got another promotion for 6 months no interest (B), you're now paying towards that (B) and payments on (A) stop. Pay that off (B) in 6 months and you go back to the original (A), but you've had no payments on it for the past few months. If you purchase another thing at 6 months (C) no interest but still have $700 left to pay off from that initial $1,000 (A) but you just got 6 months no interest on $200 (C)....the original promotion only has 5 months left, versus the new one at 6 months (C). If you only pay minimums, you're paying towards that $700 (A) remaining without even touching your new $200 (C) one. Finally, you wipe out the 1yr no interest promo (A)...and have one month to pay off $200 (C) on your third no interest promo. At least, that's how it was when I was there. When you make a payment in store, you never hear anything about your promotions. Nothing on a printout. You're supposed to check your statement, which most people never did. They'd just see their "balance due" on the screen and make a payment. You could check it online as well, but it only shows you the first soonest-expiring promotion. You have to dig a bit to check the others.
People would stack promos up and make them cut into each other, and then get mega assmad and scream at customer service reps because they got hundreds in interest due to a promotion they didn't pay attention to. Many people thought you'd pay off the lowest amount first.. I've had people yell and swear and throw things and have personally been hit by an Xbox 360, grabbed over the counter, had cards thrown at me, etc. If you know what you're doing and are fully aware of the cards and all their "promos", you can manage it pretty well and get points and all that jazz.
So, my prior post isn't "don't ever do this i got scammed!" and more a dramatic "don't do this for an impulse purchase on something you can't afford." Perhaps I was over-dramatic. Hopefully that makes sense though.