Error is throwing while selecting element of drop down

I hav a drop down inside content template  with required field validator . When I am selecting element to fill drop down of another drop down . It throws the below error.

but this error do not come every time.

pls suggest  

Unhandled exception at line 939, column 13 in http://localhost:65026/TPSInvoice/ScriptResource.axd?d=SKwhlk30vFNlxGV14kOXdnNUZ0gDU_NFmOfkpU0_sz_QvlUXfHdTnRrJq0tUm8Ny4ilg9vaNnLfyU_kDXssCUSan8UDIos2qwcYz9Zo1II5RAlbtPX84cY56nY4glnkAWZCVwTo9-ULOl5s0Esm_Sw2&t=ffffffffe4ec58b9

0x800a139e - Microsoft JScript runtime error: Sys.WebForms.PageRequestManagerServerErrorException: 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.