70-200 2.8 Do I have an issue?

Excuse me if this is a stupid question but I was always told that with this lens you could focus at 70mm and zoom in manually and the focus would still be spot on.

If I'm confusing you what I'm trying to say is that last night I was shooting a theater event and I would pick my focus point and use my back button to lock it down. If the subject was static I would then decide to zoom in without changing my distance from the subject. What I noticed is that if I picked my focus point at the 70mm range it would be out of focus if I zoomed in before the shot at 200mm ... The only reason I'm confused here is because I thought that the focal point should still be sharp when zoomed in if distance to subject doesn't change. Now I will say this, if I locked focus at 100mm or something like that it would still be sharp when I went to 200mm.

I wasn't using Servo or anything odd...

Thanks for your replies, I'm really just curious more than anything.

I think you have been misinformed, in-order to do that you need a parfocal lens.
http://en.wikipedia.or​g/wiki/Parfocal_lens

I thought so... Thanks Matthew.. Yeah, I was perplexed because I could have sworn that someone once told me that the 70-200 would hold down the focus. It really didn't make sense the more I though about it. Maybe I'm getting confused with the being able to use the manual focus ring even after auto focus.