Failed to Load ViewState Exception

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C#2.0

C#

ASP.NET

i m getting this exception i dont where have i done mistake
actualy i have a gridview, in footer i have dropdown textbox and a button on button click i m adding the dropdownselected value and textbox text in list two times when i click on button it works but when i click third time i start getting this exception

Failed to load viewstate. The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request. For example, when adding controls dynamically, the controls added during a post-back must match the type and position of the controls added during the initial request

if i turn off the enable viewstate then in mygrid only one time items are populated ...