How to stop groups from disappearing in Photoshop CS2

I have a photoshop file with many layers. I carefully arrange the layers into different groups. I work one something. Suddenly, I look at my Layers window, and all the groups have disappeared! All the layers are there, but the group tabs have gone.

What am I doing to cause this to happen? More important, how can I stop it from happening?

[It happens even when I carefully lock the layers and the groups.]

I found out when it happens. It happens when I do a merge visible.

If I have two layers, say, not sitting in any group, and these are the only layers marked as visible. I select "merge visible" from the menu, and the two layers are merged, and all my groups are gone?

Is this a bug or a feature?

I always choose layers (ctrl-click) and use ctrl-e to merge them. (Not sure what the exact menu command is, maybe "flatten down".) I've never had a problem with losing groups this way.

It's a feature.

Consulting help, use "Merge visible" to "merge layers and groups." I should have been using "merge layers".

Thanks for your reply.

Merge Visible merges all visible layers and groups. Even if you don't have the layers inside the groups visible that the groups themselves are. Just make sure to turn off the group's visibility if you don't want it merged.