Date:  04/04/2011 12:58:11 PM Msg ID:  004266
From:  FoxWeb Support Thread:  004259
Subject:  Re: Error 12. No se encuentra la variable \".
If the problem occurs when your users press the Back button, then you should probably check for problems related to you saving page-specific information in session variables or cookies. Data stored in both of these facilities is active throughout the user's session, so you should never use them to store anything that is specific to a page view. Such data should be transferred via the query string or hidden form fields.
You could try to prevent users from pressing the back button by using JavaScript to open the site in a new browser window that does not have a toolbar. However, users can still press backspace, or select back from a context menu. The best solution is to modify your code so that your site works properly when the back button is pressed.
You can trap for errors by specifying your own error handler in the FoxWeb Control Center. Errors are always trapped by FoxWeb's error handler, which logs the error, but if you specify a custom error handler, then the default error handler calls this custom handler once the error is logged. Your custom error handler could check for the type of error, procedure name, line number, etc. and display a custom message. Instead of writing a handler from scratch, I recommend that you simply modify ErrHandler.fwx, which comes with FoxWeb. Depending on the version of FoxWeb you are running, this file will either be in \Program Files\FoxWeb, or in the data folder, accessible via the FoxWeb Control Center.
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Sent by Carlos Fuertes on 04/04/2011 03:55:10 AM:
I have changed the macrosubstitution method as you sugested, but the problem remains the same.
I have realized that it arises mostly when people go back to the previous page and select another Parte_num.
First: How could I avoid people going to the previous page of the browser? 
Second: Could I catch the Error 12 in any way in order to show a messagebox to the user and redirect him to another script?
Any example would be really apreciated