Proof error found in accepted conference paper - what to do? I am a 3rd-year computer science PhD student and I have recently written and submitted a paper to a theoretical conference. It has been accepted; but the reviews were not very in-depth. Usually they were of the form:
"I didn't check the proofs but the statement sounds reasonable."

Now I have to say that we work on a very complicated topic in which there are maybe just a few experts worldwide; and even for those it would have probably taken them a few weeks if they were to check the proofs in depth. In light of this I was not very surprised that most of them just took a quick glance.

While preparing the presentation, however, I myself noticed a mistake which is not easily solvable. I did find a solution, however, that keeps the main proof structure intact, but one of the main definitions has to be changed.

I asked my professor what we should do about the paper, and he said to leave it be and publish the corrected version in a journal later. However, I still feel kind of uneasy pretending as if nothing happened. What would be the ethical approach? I realize that it is probably not possible to "stop the presses", but should I notify the conference organizers about it in any way? Or should I mention it in my talk?
This is what errata are for.

I had something like this happen to me with one of my early papers.  For that paper, I knew the result was likely to be significant, so I knew I did not want to let the paper stand with an error in it.  It was too late to correct for the conference proceedings, but I could correct the archival version that ended up in the ACM Digital Library, and I did.

Every (reputable) publisher has its own procedure for dealing with errata; you just need to figure out what it is and follow it.  Following this process is likely to be somewhat long and annoying, but if it's important to you to get it right, there's no reason not to do so.