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We may get "The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again." issue when try to BreakRoleInheritance  of list programmatically. To resolve this, Add AllowUnsafeUpdates = true.

For Example,

web.AllowUnsafeUpdates = true;

Make it false once BreakRoleInheritance is over.