first row or fails with an error:
  ERROR 1022 (23000): Can't write; duplicate key in table ''

The server uses intermediate temporary table to store updated
row data.  The first column of this table contains rowid.
Current server implementation doesn't reset NULL flag of that
column even if the server fills a column with rowid.
To keep each rowid unique, there is an unique index.
An insertion into an unique index takes into account NULL
flag of key value and ignores real data if NULL flag is set.
So, insertion of actually different rowids may lead to two
kind of problems.  Visible effect of each of these problems
depends on an initial engine type of temporary table:

1. If multiupdate initially creates temporary table as
a MyISAM table (a table contains blob columns, and the
create_tmp_table function assumes, that this table is
large), it inserts only one single row and updates
only rows with one corresponding rowid. Other rows are
silently ignored. 

2. If multiupdate initially creates MEMORY temporary
table, fills it with data and reaches size limit for
MEMORY tables (max_heap_table_size), multiupdate
converts MEMORY table into MyISAM table and fails
with an error:
  ERROR 1022 (23000): Can't write; duplicate key in table ''


Multiupdate has been fixed to update the NULL flag of
temporary table rowid columns.