Error with debug="true"

[Gianni Nitti]

If I set in web.config this:
<compilation defaultLanguage="vb" debug="false"/>
I receive this error:

"Compiler Error Message: BC30560: 'barratop_ascx' is ambiguous in the
namespace '_ASP'.
Source Error:
Line 47: <table height="100%" cellSpacing="0" cellPadding="0"
width="100%" border="0">
Line 48: <tr>
Line 49: <td vAlign="top" height="86"><uc2:barratop id="Barratop1"
runat="server"></uc2:barratop></td>
Line 50: </tr>
Line 51: <tr>"

Instead, if I set:
<compilation defaultLanguage="vb" debug="true"/>
it's all ok, no errors.

Why???
What I have to do?