>There is a setting called ValidateRequest that you can set to turn this feature on and off. The setting is in Machine.config, web.config, and can be set in the @ Page directive.
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>There is not an event hook for this, but it throws an HttpRequestValidationException that you can intercept in a Try/Catch block. I'm not too sure where to put the Try/Catch. You can also look for the error in the Global.asax Application_Error event.

After looking more at what causes this, I found that the memo field contains some HTML characters such as B and BR. .NET handles those characters automatically on form display and postback if we enter them as encoded. This resolves the issue. I do not feel ok to workaround this issue by changing the top level and opening that door. I made some tests. If I enter the opening character of a tag as encoded, it is saved ok and shown ok on the way back. I will have a procedure which will apply some conversions on the client data once we go in production. Thanks