Here is a case where libmutt destroyes the context behind us (I think)

[Emmanuel <e allaud wanadoo fr>]

	Hi all,
in mx_sync_mailbox : look at the following line :

      if (ctx->msgcount == ctx->deleted &&
	(ctx->magic == M_MBOX || ctx->magic == M_MMDF) &&
	!mutt_is_spool (ctx->path) && !option (OPTSAVEEMPTY))
      {
        unlink (ctx->path);
        mx_fastclose_mailbox (ctx);
        return 0;
      }

This is not undefed by LIBMUTT so we use this code. It seems that in the 
case you
have deleted all messages in a mailbox (ctx->msgcount==ctx->deleted) and
that mailbox is of type MBOX or Maildir libmutt can destroy the context
behind us. The only fact I'm not sure if for the flags OPTSAVEEMPTY. We
don't set it in libbalsa/libbalsa.c, but I don't know his default.
Hope it helps.
Bye
Manu