incremental_sweep_phase(mrb_state *mrb, mrb_gc *gc, size_t limit)
{
  mrb_heap_page *page = gc->sweeps;
  size_t tried_sweep = 0;

  while (page && (tried_sweep < limit)) {
    RVALUE *p = objects(page);
    RVALUE *e = p + MRB_HEAP_PAGE_SIZE;
    size_t freed = 0;
    mrb_bool dead_slot = TRUE;
    mrb_bool full = (page->freelist == NULL);

    if (is_minor_gc(gc) && page->old) {
      /* skip a slot which doesn't contain any young object */
      p = e;
      dead_slot = FALSE;
    }
    while (p<e) {
      if (is_dead(gc, &p->as.basic)) {
        if (p->as.basic.tt != MRB_TT_FREE) {
          obj_free(mrb, &p->as.basic, FALSE);
          if (p->as.basic.tt == MRB_TT_FREE) {
            p->as.free.next = page->freelist;
            page->freelist = (struct RBasic*)p;
            freed++;
          }
          else {
            dead_slot = FALSE;
          }
        }
      }
      else {
        if (!is_generational(gc))
          paint_partial_white(gc, &p->as.basic); /* next gc target */
        dead_slot = FALSE;
      }
      p++;
    }

    /* free dead slot */
    if (dead_slot && freed < MRB_HEAP_PAGE_SIZE) {
      mrb_heap_page *next = page->next;

      unlink_heap_page(gc, page);
      unlink_free_heap_page(gc, page);
      mrb_free(mrb, page);
      page = next;
    }
    else {
      if (full && freed > 0) {
        link_free_heap_page(gc, page);
      }
      if (page->freelist == NULL && is_minor_gc(gc))
        page->old = TRUE;
      else
        page->old = FALSE;
      page = page->next;
    }
    tried_sweep += MRB_HEAP_PAGE_SIZE;
    gc->live -= freed;
    gc->live_after_mark -= freed;
  }
  gc->sweeps = page;
  return tried_sweep;
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.