Apple store wouldnt replace my phone because they said a third party had changed the part with the serial number inside my phone, i know for a fact that i did not use a third party to fix my phone, why and how could this of happened?

I recently took my 4S into the apple store seeing as I had dropped it without a case one too many times. I knew I would do this so I bought apple care to ensure that even though my phone broke it would be somewhat covered. When I went into the store they took my phone apart and told me a yellow sticker was on the inside of the phone which PROVED that i had taken my phone (under warrenty) to a third party for them to fix. I know for a fact i did not do this and they proceeded to say how my warrenty was completely voided because the serial on the inside of the phone did not match the serial through itunes. I bought my phone brand new from the apple store, they took it out of the box in front of me and turned it on and everything, i know for a fact this phone was not refurbished or used in any way when i bought it. I did not take it apart myself, I did not take it to any third party people, and literally the only time my phone was out of my possession when i bought it was when they took it in the back room to take it apart. Can ANYONE shed any insight as to why this happened and is anyone else having this issue?