Found a severe error in a conference paper after presentation but before the proceedings
Let's see:


You wrote a paper of sufficient writing quality that it was chosen for presentation at a conference and publication.
None of the peer reviewers noticed anything wrong with it.
None of the people in the audience questioned it.
Your supervisor saw nothing wrong with it.
You gave an excellent presentation.
You found a flaw in a paper that had already been accepted for publication.
You had the integrity to withdraw your paper from publication.


That's a long list of things to put on your "plus" list.
Most undergrads don't achieve even one of them.
None of them should be seen as a negative.



Be warned that you'll become more jaded as you progress.

I remember as a grad student being asked by my supervisor to review a paper that had been sent for him to review.  It was well written and obviously the result of a lot of hard work, but I found a fundamental oversight that would have allowed the entire problem to be reduced to something very trivial.

We told the publishers about this, and they wrote back requesting that we recommend it for publication anyway.  My supervisor said that the author could publish another paper later that would re-analyze the problem and present it differently.

That's one (of many) reasons I eventually didn't bother finishing my own PhD.