Sporadic Invalid postback or callback argument Error

All,
We have a login page on load-balanced servers that will occasionally, and I mean very occasionally throw this error:
Error Description: Invalid postback or callback argument.  Event validation is enabled using <pages enableEventValidation="true"/> 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. Source: System.Web Stack Trace:    at System.Web.UI.ClientScriptManager.ValidateEvent(String uniqueId, String argument)    at System.Web.UI.WebControls.TextBox.LoadPostData(String postDataKey, ...