I have been recently working on my Master thesis. The idea for it was approximately the following: take a recent Nature paper of my supervisor, reproduce the results and add them to my Master thesis. This is exactly what I did, and I improved the experimental setup significantly, as compared to previous studies.
Unfortunately, during the final stages of the thesis I became increasingly confident the paper my work was based on contained a critical error. As I am an experimentalist, and rigorous investigation of this topic required a considerable amount of non-trivial theoretical work, I decided to stick to the original methodology of the paper in my thesis in the remaining time, while highlighting the possible problem in a suggestive way. I discussed this with my supervisor, and he agreed this might have been the case.
This year, I applied to US grad schools in physics, and I got into a top-1 program in my field. I will probably have to present my results to those people in order to get into the lab I like. As these people are literally the best in the world, I am sure they will spot a mistake, if there is one, and I am 99% positive this is the case.
Though the people from that lab seemed to like me initially very much (I had some other results to brag about), I am worried their opinion might change if I present this questionable methodology I used.
My question is: if the questions arise during my presentation, how can I discuss the issue in a tactful manner, being an excellent representative to my previous workplace?