lattice: packet.number() versus panel.number()

Hi,

According to
https://svn.r-project.org/R-packages/trunk/lattice/R/print.trellis.R,

   "[panel.number] is usually the same as, but can be different from
packet.number"

and I had been under the impression that as long as the user is not
using a custom index.cond nor perm.cond, the panel.number would in fact
be the same as the packet.number.

However, I have recently come across a case where the two are *not* the
same, even though I am not using a custom index.cond nor perm.cond.

So my question is, what might be some other possible situations in which
the two would be expected to differ?