According to http://www.opengl.org/sdk/docs/man/xhtml/gluTessCallback.xml (don't know if this is official or not.):

void combine( GLdouble coords[3], void *vertex_data[4], 
              GLfloat weight[4], void **outData );
                        
All vertex pointers are valid even when some of the weights are 0. coords gives the location of the new vertex.

The following example code is also given:
void myCombine( GLdouble coords[3], VERTEX *d[4],
                GLfloat w[4], VERTEX **dataOut )
{
   VERTEX *new = new_vertex();

   new->x = coords[0];
   new->y = coords[1];
   new->z = coords[2];
   new->r = w[0]*d[0]->r + w[1]*d[1]->r + w[2]*d[2]->r + w[3]*d[3]->r;
   new->g = w[0]*d[0]->g + w[1]*d[1]->g + w[2]*d[2]->g + w[3]*d[3]->g;
   new->b = w[0]*d[0]->b + w[1]*d[1]->b + w[2]*d[2]->b + w[3]*d[3]->b;
   new->a = w[0]*d[0]->a + w[1]*d[1]->a + w[2]*d[2]->a + w[3]*d[3]->a;
   *dataOut = new;
}

Implying that all vertex pointers are indeed valid.

However, in the function

static void SpliceMergeVertices( GLUtesselator *tess, GLUhalfEdge *e1,
				 GLUhalfEdge *e2 )
/*
 * Two vertices with idential coordinates are combined into one.
 * e1->Org is kept, while e2->Org is discarded.
 */
{
  void *data[4] = { NULL, NULL, NULL, NULL };
  GLfloat weights[4] = { 0.5, 0.5, 0.0, 0.0 };

  data[0] = e1->Org->data;
  data[1] = e2->Org->data;
  CallCombine( tess, e1->Org, data, weights, FALSE );
  if ( !__gl_meshSplice( e1, e2 ) ) longjmp(tess->env,1);
}

data[3] and data[4] are NULL. Thus if an application expects the pointers to be valid, they will crash.

A possible fix:

  data[0] = data[2] = e1->Org->data;
  data[1] = data[3] = e2->Org->data;