I think this is a huge no-no. I've heard that it could be grounds for revoking acceptances, since this is considered an 'honor' violation. People on this forum have even talked about horror stories of being a 3rd or 4th year, and getting kicked out then when it is finally figured out.
If it was an honest mistake - bring it up as soon as possible. Better you bring it up rather than someone else. People do make mistakes - I don't think you would be faulted for this. For example - you take one class at a community college years ago and just forget to put it in your AMCAS. The class doesn't hurt your overall GPA, so it's easy to explain it as just forgetting.
On the other hand, if it's obvious you are trying to hide something - like you have a .5 GPA from your first 2 years of undergraduate and just didn't want schools to see it - good luck. I don't know how much mercy you would be shown in that case.
-chop