It depends on where the validation is happening.  If it is happening on
the clientside, then the page shouldn't post back if it is not valid.
What are you trying to do with in your On_Click handler? (ie add items
to viewstate/session).

I had a problem once where I had something happening similar where I
was putting items into the Session and then I had a function that would
load all the items from session each time the page loaded.

What I did once I realized that it would try to bind to late, was I had
to do all my binding to the grid in the On_Click as opposed to the
Page_Load because the controls would get their data from the view state
and such before the Page_Load would happen.

I don't know if this might be a problem that you are having, but you
might want to look at the page life cycle.
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cp
concontrolexecutionlifecycle.asp

-Darren Kopp
http://blog.secudocs.com/