traveler982 said:
What should I say? That I forgot that trip and don't remember the right dates?
Correct. Explain the situation that you traveled to a neighbour country where you didn't need to show your passport (state the reason why you didn't have to show your passport e.g. it was within the EU or so, whatever it was in your case) but for completeness reason you want to list it.
CIC asks for all that information because they check with every security agent of each country in which you traveled in if there is any criminal file or something else.
I have a EU passport and traveled within Europe without showing passport at all. ID was good enough. Nevertheless I listed all trips to Italy, Spain, Denmark etc because all countries needed to be listed in travel history.
Which country did you miss to list? Just curious if it is a "dangerous" country. That might result in extra documents for an extended security check.