WiredTiger should never attempt to directly insert a birthmark into lookaside but rather convert the update matching the on page update to a birthmark and then insert it.

We find the error on line 735, of cache_las.c where we're walking an update chain and land on a birthmark to insert into lookaside, this would imply that we're inserting updates from the wrong page image or that when we instantiated the page back out of lookaside we didn't replace the birthmark with the on page image. Its also possible that something else went wrong.

(gdb) p *page->modify->u2->row_leaf->update[3]->next->next->next->next->next->next->next
$24 = {
  txnid = 1248522,
  durable_ts = 6818933148508752372,
  start_ts = 6818933148508752372,
  next = 0x7fe55dc1ac00,
  size = 0,
  type = 1 '\001', #WT_UPDATE_BIRTHMARK
  prepare_state = 0 '\000',
  data = 0x7fe55f94efe6 ""
}