ALTER TABLE crashes
Executing fast alter table (one that doesn't need to copy data)
on tables created by mysql versions prior to 4.0.25 could result
in posterior server crash when accessing these tables.

There was a bug prior to mysql-4.0.25. Number of null fields was
calculated incorrectly. As a result frm and data files gets out of
sync after fast alter table. There is no way to determine by which
mysql version (in 4.0 and 4.1 branches) table was created, thus we
disable fast alter table for all tables created by mysql versions
prior to 5.0 branch.
See BUG#6236.


sql/sql_table.cc:
  There was a bug prior to mysql-4.0.25. Number of null fields was
  calculated incorrectly. As a result frm and data files gets out of
  sync after fast alter table. There is no way to determine by which
  mysql version (in 4.0 and 4.1 branches) table was created, thus we
  disable fast alter table for all tables created by mysql versions
  prior to 5.0 branch.
  See BUG#6236.