Interspire Email Marketer have a special custom field/variable that can be used to display error to your contacts when they encountered errors while using your "Web-Forms".
For internal error pages that gets stored in Interspire Email Marketer internally, you can use the custom field
%%GLOBAL_Errors%% in order to display these error messages to your contact...
For external error pages where you "use existing URL" for your error pages, Interspire Email Marketer passes a special variable in the URL called "Errors".
An example of an external error page code:
============================================================================
<html>
<head><title>Error has occurred</title></head>
<body>
The following error has been returned: <?php print $_GET['Errors']; ?>
</body>
</html>
============================================================================

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