Invalid postback error

[Samuel]

Invalid postback or callback argument. Event validation is enabled using in
configuration or <%@ Page EnableEventValidation="true" %> in a page. For
security purposes, this feature verifies that arguments to postback or
callback events originate from the server control that originally rendered
them. If the data is valid and expected, use the
ClientScriptManager.RegisterForEventValidation method in order to register
the postback or callback data for validation.

I often get this error after changing the asp files

What can be done to avoid it?

thank you,
Samuel