I don't like it, but I know it does happen, however, they usually have an out in their rules, even when you did everything right.
I use the search term "Sponsor reserves the right" if I run into a problem.
Sponsors/JA's use boilerplate verbiage like:
"Sponsor reserves the right to cancel/terminate, modify or suspend the Sweepstakes if in Sponsor’s sole determination it believes that the integrity of the Sweepstakes has been, or could be, compromised or that the Sweepstakes is not capable of running as planned or fairly, including, but not limited to, fraud or force majeure or any other causes beyond Sponsor’s control."
Which can cover a lot.
I usually just hear about it but I got just hit by BIC when I got a congrats screen & could have won:
Weekly {3/18-4/21} Instant win Prizes {54 per week-1ppp/day} ; {1} JamboxTM Speakers. ARV: $180. {3} iPod® Nano. ARV: $150. {6} $40 Cash. ARV: $40. {7} $50 iTunes® Gift Card. ARV: $50. {9} Lava Lamp. ARV: $20 . {12} BIC® Birthday lg T-Shirt. ARV: $10. {16} Bottle Koozie & Opener Package. ARV: $5.
Instead, I got an email that said: "...to show our appreciation for your participation and patience, we will be sending you a high-value coupon (Free up to $4.99) to the mailing address you provided when you registered for the Promotion."