Call Apple back, and explain to them your situation.
admit to them that you made a mistake and that your sorry. Tell them your willing to go to the apple store, and let a genius look at your mac pro. To verify that it is in good working order. If your nice about it, they may let you slide.
Yes, sometimes that works.
And, sometimes, depending on how much Apple (or a given sales rep) is hurting for the sale, they may contact you.
I've let the warranty lapse on a few Apple items, and received a phone call one week prior to it lapsing and another one the day it was to lapse (and I think on one occasion I got a call after it lapsed).
They can over-ride it. But, they are not obligated to.
I ordered AppleCare for my wife's iBook about 10 days before her warranty was to expire. I was going to have to wait for it to arrive in the mail. And, it was quite possible that it would not arrive in time. So, I asked them if they could insure that I would be able to use it. They put a note in my file to help if the package did arrive late. And, fortunately I received it and was able to register it just in time. I think it was one day before the expiration that I was able to register the code and extend (the package got here just in time).
Now, if I had chosen to buy the AppleCare extension at retail, I could have registered it right away. But, I ordered it to save money (since we get a corporate discount).