I write this and it´s ok:
<ItemTemplate>
<a href="" onclick="javascript:window.open('AclaracionIncidencia.aspx?codigo=<%#
Eval("codigo") %>','NewPage','width=300,height=300');return
false;"><%# Eval("motivo")%></a>
</ItemTemplate>
With return false,
not execute click.
Sorry, i don´t know why, but i can´t write # inside href. Change text and put a long url....You must write # in href.