When inserting into a table with an AUTO_INCREMENT column and inserting the function name now() it fails and causes the auto_increment value for the table to jump to the maximum for the data type. No further records can be inserted until the rouge rows are deleted and the auto_increment is reset. (Not a common mistake to make in scripting, but can happen if inserted fields are in the incorrect order.)

CREATE TABLE `T_test` (

  `ID` int(11) NOT NULL AUTO_INCREMENT,

  PRIMARY KEY (`ID`)

) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=latin1;

insert into T_test values (now());

1 row(s) affected, 1 warning(s): 1264 Out of range value for column 'ID' at row 1

2147483647

Does the same for LOCALTIMESTAMP() & LOCALTIME()

select AUTO_INCREMENT from information_schema.TABLES where TABLE_NAME='T_test';

2147483647

set @nowtime=now(); insert into T_test values (@nowtime);  

does not give the error.