Is it wrong to unintentionally withhold information? I recently posted something related to this (If an assumption proves to be false, is the research then meaningless?), but it is still bothering me. 

As I mentioned in that last post, I conducted my first research project in high school last year, and I am now having doubts about one of my assumptions (I was not used to making assumptions at the time). I actually learned that one of my assumptions were invalid from a judge at a science fair.

As a matter of fact, I had not even noticed that I had made that assumption until he brought it up (and penalized me for it). Despite all of the people that I had proofread and check my research, no one brought it up. Furthermore, I competed at science fairs prior to that one and none of those judge brought it up (I am not sure if they caught it or not). I was successful at those science fairs, but I am now feeling guilty. I feel as if me not mentioning that one assumption is the only reason I was as successful as I was (although I cannot justify it).

Is it wrong that I withheld this information, despite the fact that I was not even aware of it? Is that considered cheating? This question has been bothering me and I would appreciate any help. (I am now working on a different research project and I am being much more cognizant about my assumptions.)
If you were not aware, you did not "withhold" because that requires intention. 

With time, you will practice insight about what implicit assumptions you make - this is part of the scientific development and even great scientists have not been free of that. Galilei transformation, for instance, was taken for granted for centuries, which was initially fine, but was sustained to the point where "ethereal" concept-bending had to be introduced to keep it alive. 

As Einstein said: "The fish is last to discover water." Do not be too hard on yourself, as long as you put it firmly into your agenda to do your best to be fully aware of your assumptions and their validity. Discussion with friends/colleagues can help here.