Okay.. maybe I am missing something. My partner and I have a problem with sudden phantom pieces.
I rez a box.. make it 10 x 10.. then copy selected to make more boxes. .. and make more until I make a building. Link the parts and call it an Object. My partner does not own any pieces nor are there any textures on them.
All the pieces are set, by the numbers to placement. I walk across my new floor and suddenly I fall thru it. AS if the prim is phantom or a piece of the prim is. Phantom is NOT checked on the object or the prim but I fall through anyway.
Hi... I reported this a bit ago...and this..was the responds answer I had received.. hope it helps

Recently, you've reported that you were having issues prims going
phantom. There are two types of issues regarding phantoming prims.
Hopefully one of these two explanations will address your issue.
First issue:
Parent/root prim of an object going phantom after being resized.
Repro sample:
- create a cube prim, goto Object tab of Edit window
- under Size, resize its dimensions to x: 10.0, y: 10.0, z: 0.010,
creating a big flat panel
- shift drag (hold down SHIFT, then drag one of the axis arrows)this
prim and create 3 more of this prim (or hit CTRL-D to make duplicates)
- place prims side by side, and link them together (left-click on each
prim while holding down SHIFT key...after noting that all 4 prims are
highlighted, go to TOOLS on the menu bar, then select LINK)
- select Edit Linked Parts, then select Stretch
- left click on the parent/root prim of the now linked object (it's the
prim that is highlighted with a yellow glow around it, as opposed to
the
white highlight of the child prims)
- resize the x & y coordinates under Size to 5.0, or drag both the red
and green scaling boxes to make the panel smaller
- close Edit window
Results:
In some cases when the parent prim of a linked object that's made up of
at least 3 prims is resized, the parent prim becomes phantom. This is
due to Havok's calculations getting messed up and how thin the parent
prim is.
Workaround: If the prim (along it's Z axis) is made thicker by at
least
0.100, phantom usually doesn't occur. You can also unlink the object
and
relink the object together after resizing your parent prim...OR unlink
one other child prim of the object and then relink that child prim back
to the object so that it's now the parent/root prim of the object and
not the prim you had resized.
Second issue:
Placing a prim across a sim border (ie. a floor panel intended as a
walkway) will have issues of phantom. Per Ben Linden:
A prim can not exist on two sims at once. All of the physics
calculations are done on the sim, and we don't have a good fast way of
communicating collision data over sim borders - so you can only collide
with things that are on the same sim as you are. When people build on
sim edges, what often happens is a prim will be overlapping the sim
border - it will look like you should be walking on it - but in
reality,
that prim is on another sim, & the sim you are on has no responsibility
to make sure you stay in the air. So you fall through (phantom). When
we build Linden Roads, we do a couple of tricks to make sure that there
is always a prim on the same sim you are on. This involves overlapping
the border with a few alpha'd reinforcements - but it means you won't
fall through.
The phantom issue explained do not always occur consistently, but thus
far, the repro provided works consistently. The issues with Havok will
hopefully be rectified when Havok 2 is implemented in the near upcoming
releases of SL (possibly 1.7), but in the meantime residents will need
to utilize the current workarounds or work within the current
limitations in SL at this time. Understand that as SL is updated &
revised to meet new specs and scaling growth, there will be issues
arising unexpectedly. As much as we try to test for obvious glitches,
covering every possible permutation of an open ended environment we
created is currently improbable. Feel free to IM me or any Liaison
Linden for further assistance if you're still having issues. Thank you
very much for your patience regarding this matter.