Sql Server Incorrectly Functioning

I have a .NET application with a sql server database backend. Our
client has been having problems lately with data being returned from
the database with wrong data. We have error logging in our stored
procedures and even this is reporting wrong. In every stored procedure
we have the following:

IF @@Error <> 0
BEGIN
EXEC dbo.HandleError
@ErrorNum = @@Error,
@ProcID = @@PROCID
END

Then, HandleError looks like:

CREATE PROCEDURE dbo.HandleError
@ErrorNum int,
@ProcID int
AS
/************************************************** *****************
* PROCEDURE: HandleError
* PURPOSE: Handles logging an error
* CALLED FM: Other stored procedures
************************************************** ******************/

SET NOCOUNT ON

DECLARE @UserID int
DECLARE @Host varchar(50)
DECLARE @Len int
DECLARE @SPName VARCHAR(100)
DECLARE @ErrorMsg varchar(500)
DECLARE @ErrorTrace varchar(4000)

-- Get our host name
SET @Host = HOST_NAME()
SET @Len = LEN(@Host) - CHARINDEX(':', @Host)

IF @Len > 0 AND CHARINDEX(':', @Host) > 0
SET @UserID = RIGHT(@Host, LEN(@Host) - CHARINDEX(':', @Host))
ELSE
SET @UserID = NULL

SET @SPName = OBJECT_NAME(@ProcID)

SET @ErrorMsg = 'SQL Error: ' + @SPName

SET @ErrorTrace = 'Error: '
+ CAST(@ErrorNum AS VARCHAR(50))

EXEC dbo.InsertApplicationErrors
@ErrorMessage = @ErrorMsg,
@ExceptionMessage = '',
@ErrorStackTrace = @ErrorTrace,
@UserID = @UserID,
@HostID = @Host,
@Logfile = '';

SET NOCOUNT OFF

GO

InsertApplicationErrors inserts a log into a table we have that we
monitor. We got an error back in the ErrorStackTrace column with
'Error: 0'. That shouldn't happen, as the only time we log error is if
it is not 0.

Does anyone have any ideas on what might be wrong?

Steve