Getting strange error in system log, why ?

I often get an error message in system log against one page, what could be the reason of this error ? and how to fix it or stop this error message ?

Log type:   Unknown

Severity:   11

Message:   Exception of type 'System.Web.HttpUnhandledException' was thrown.

Exception:   System.Web.HttpUnhandledException: Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.ArgumentException: 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. at System.Web.UI.ClientScriptManager.ValidateEvent(String uniqueId, String argument) at System.Web.UI.Control.ValidateEvent(String uniqueID, String eventArgument) at System.Web.UI.WebControls.DropDownList.LoadPostData(String postDataKey, NameValueCollection postCollection) at System.Web.UI.WebControls.DropDownList.System.Web.UI.IPostBackDataHandler.LoadPostData(String postDataKey, NameValueCollection postCollection) at System.Web.UI.Page.ProcessPostData(NameValueCollection postData, Boolean fBeforeLoad) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace --- at System.Web.UI.Page.HandleError(Exception e) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest() at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) at System.Web.UI.Page.ProcessRequest(HttpContext context) at ASP.requestaquote_aspx.ProcessRequest(HttpContext context) in c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\49f8cc15\b725da54\App_Web_zg9kmljm.18.cs:line 0 at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)