It is a nuance of the language. The 'did you' puts this in the past so 'see' would be the best choice. The answer would be 'I saw it once' but equally correct would be 'I did see that once'. At any rate the meaning was clear and that is the important thing. While English is the language of the board actual native English speakers are almost certainly in the minority. Conveying the meaning is the only thing that should be considered important here, not using the King's English. (It is more important if you are posting to your site or blog and hoping to reach a native English speaking audience, but aware surfers understand that native English speakers are a minority.)