Harvard will not find out. In fact, it would be against privacy laws to disclose anything about you unless you actually worked there. And in that case, you would have to tell Harvard about working there before they could even contact HR to confirm employment. So there is a 0% chance that Harvard will find out. Plus, you really arn't doing anything wrong, just bad form. I would call the HR department or representative that you've been in contact with and let them know that you recieved another offer with a top consulting company and will be taking it. Apologize and send a thank you letter (hand written) to all people that you interviewed with and the HR persons to leave it on the best terms possible.
If you recieved any signing bonus, compensation, or anything else you will need to return it. You should also be somewhat apologetic, but also just tell them exactly what you wrote. Anyone in business will understand that you got a better offer, and took it. THis stuff happens in business, but it is technically bad form to reneg after you've signed an offer. But if you don't want them, trust me, they don't want you. They are wasting valuable resources and time, so they will appreciate your honesty. Also, if they were interviewing multiple candidates, HR procedures usually have a backup plan for this happening. They don't tell the #2 or #3 candidates yes or no until the day you start. So at this point, the offer will most likely move onto the next in-line candidate, the position will not be filled, or they will pull the position all together.
In life, you will have these decisions to make, and it sounds like you must have an impressive resume to get 2 internship offers. So congratulations to you, and good luck with 2+2 applications. Top consulting offer will definitely be highly favored over the credit card company for those types of programs. Just make sure to be the BEST intern they have, and you might be the perfect candidate.
New to the GMAT Club? <START HERE>
My GMAT and BSchool Tips: