Axiom - Jun 23, 2013 7:51
Can't say what this is for sure since I don't completely understand source, but it seems to involve instances.
0x080cffa1 in pc_setpos (sd=0xb648490, mapindex=840, x=99, y=29, clrtype=CLR_OUTSIGHT) at pc.c:4716
... if( map[m].flag.src4instance ) {