I ran into a problem where the CustomValidator OnServerValidate not Firing. After a lot of trial-and-error checking, I found out, the problem was that it didn't tolerate the yellow brackets. No errors were thrown, which was very frustrating. This is what didn't work, to make it work, all i had to do was move the error to the code-behind: Normal 0 false false false EN-US X-NONE HE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; ......