When transferting data from one RadListBox to another the “Sys.WebForms.PageRequestManagerServerErrorException: Index was out of range. Must be non-negative and less than the size of the collection.” is thrown.
This error occurs when the state of the two RadListBox controls is not yet updated after transferring/reordering items. By setting the AutoPostBackOnTransfer property to True, you could ensure that PostBack is performed after each action is made. Thus you do not need to rely on the state of the controls and you could easily avoid the exception.