(saw your problem in the channel, figured I'd give my two cents, although given the trouble I had with simple IB stuff recently, I'm not sure that this post isn't even worth that [smile])
If you have a rectangular strip that's arranged like this:
024135
Then your IB will be as follows, for example:
021 123 243 345
which links the verts together.
However, if you add, say, 1 to those indices, you'll get:
132 234 354 456 (let's assume we have a 6th vert laying around somewhere).
As you can see, even though the vert groupings are the same to make the triangle, they ended up flipping the direction of the cycle (went from CW to CCW). If it did something like that for a relatively simple VB/IB relationship, I can only imagine the havoc it would wreak on a more complex setup where, for example, the verts aren't laid out in a fairly sequential fashion. Not to mention what would happen if a 6th vert didn't even exist in the VB!
What exactly was your logic behind doing this? Perhaps that in and of itself was flawed.
Disclaimer: I'm not hugely familiar with DIP's offset, so I'm mainly going off of the logic of SetStreamSource.