Oddity with BDB backend

[Mike Whitaker <mike@mindcandydesign.com>]

Trying to do a MOD on an attribute on a node very occasionally  
returns me this (slapd v2.2.23)

> Oct  5 10:44:35 7s8xs1j slapd[10425]: conn=3 op=3 MOD  
> dn="cn=dave,ou=users,dc=<withheld>"
> Oct  5 10:44:35 7s8xs1j slapd[10425]: conn=3 op=3 MOD attr=reminder
> Oct  5 10:44:35 7s8xs1j slapd[10425]: conn=3 op=3 RESULT tag=103  
> err=80 text=context csn store failed

After this, it becomes impossible to bind as cn=dave...

Odder still, if I do:
/etc/init.d/slapd stop && slapindex && /etc/init.d/slapd start
everything seems to sort itself out.

Checking the source suggests that the error comes from ctxcsn.c, in  
one of two places, with code of the form
         ret = bdb_id2entry_add( op->o_bd, tid, *ctxcsn_e );
                switch ( ret ) {
                case 0 :
                        break;
                case DB_LOCK_DEADLOCK :
                case DB_LOCK_NOTGRANTED :
                        goto rewind;
                default :
                        rs->sr_err = LDAP_OTHER;
                        rs->sr_text = "context csn store failed";
                        return BDB_CSN_ABORT;

(or the same or similar for bdb_dn2id_add() ).
It doesn't happen every time.

Any suggestions, help, advice?