When you have defined a vertex group and in edit mode you hide part of that vertex group, but don't hide another part of the same vertex group the following will happen:
As soon as you select the vertex group and then delete selected vertices, then unhide your hidden geometry also your hidden vertices that belonged to the vertex group are deleted.

Not entirely sure, but I don't think this is supposed to work like this.
It feels very inconsistent, because with other functions, for example transforms, the hidden vertices are never affected and I always thought hidden vertices are 'protected', so nothing could happen to them as long as they are hidden.

I have attached a file (with instructions) which demonstrates the problem.


I don't think my system specs will matter for this, but here they are:

Mac OS X 10.5.8
NVIDIA GeForce GT 120
Blender 2.56, also tested under Blender 2.49a


Thanks for looking into this. And apologies if it is indeed supposed to work like this.

Timo