As of K2 smartforms 4.6.9, including all subsequent releases, Microsoft Internet Explorer 8 (including any later Internet Explorer version running in IE8 compatibility mode) is no longer supported. As a result you could experience the following issues:
Issue 1: When running a SmartForm in IE8 or earlier compatibility mode, you'll receive an error which blocks the Form from displaying in the browser. 
Issue 2: When browsing a SharePoint 2010 or SharePoint 2013 in SharePoint 2010 mode site and running K2 4.6.9 or later with Internet Explorer in IE8 document mode, the K2 Forms Viewer web part form may not render or you will see an infinite spinner.

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If the browser is not configured to display errors the infinite spinner is displayed:

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 In SharePoint 2010, the master page document mode is set to IE8 (the default setting), but this results in the K2 Forms Viewer web part not functioning as expected in Internet Explorer 8 and IE=8 document mode as the support for both the browser and the mode is deprecated.

The table below describes the possible scenarios you could have when using IE8. Use the Resolutions column to address the issue.

scenario Coldfix/Product Resolutions Message Presented
1. Design Time - Running the K2 smartforms designer in IE8 or IE 8 compatibility mode is unsupported. Has never been supported • Use a later version of Internet Explorer.
• Use a different browser.
• Disable IE8 compatibility mode.
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2. Runtime - Running any K2 smartforms Form directly in IE 8 is unsupported. From 4.6.9 • Use a later version of Internet Explorer.
• Use a different browser.
• Prevent K2 smartforms from running in Compatibility mode or  Enterprise mode in Internet Explorer by using Edge Mode – see KB001704 on how to configure Edge Mode.
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From 4.6.10 • Use a later version of Internet Explorer.
• Use a different browser.
• Prevent K2 smartforms from running in Compatibility mode or Enterprise mode in Internet Explorer by using Edge Mode – see KB001704 on how to configure Edge Mode.
• Enable legacy Browser Support – See How to set  the Forms.LegacyBrowserSupport application setting to "true" in the site's web.config below for further details on this option.
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3. Running the K2 smartforms Form (runtime) in a Browser with IE 8 compatibility mode is unsupported. From 4.6.9 • Use a later version of Internet Explorer.
• Use a different browser.
• Prevent K2 smartforms from running in Compatibility mode or  Enterprise mode in Internet Explorer by using Edge Mode – see KB001704 on how to configure Edge Mode.
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From 4.6.10 • Use a later version of Internet Explorer.
• Use a different browser.
• Prevent K2 smartforms from running in Compatibility mode or Enterprise mode in Internet Explorer by using Edge Mode – see KB001704 on how to configure Edge Mode.
• Enable legacy Browser Support – See How to set  the Forms.LegacyBrowserSupport application setting to "true" in the site's web.config below for further details on this option.
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4. Running the K2 Forms Viewer web part in SharePoint 2010 where the master page document mode is set to IE=8 by default. Broken in 4.6.9 • Use a later version of Internet Explorer.
• Use a different browser.
• Prevent K2 smartforms from running in Compatibility mode or Enterprise mode in Internet Explorer by using Edge Mode – see KB001704 on how to configure Edge Mode.
• Get the available cold fix from Regional support.
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From 4.6.10 • Use a later version of Internet Explorer.
• Use a different browser.
• Prevent K2 smartforms from running in Compatibility mode or Enterprise mode in Internet Explorer by using Edge Mode – see KB001704 on how to configure Edge Mode.
• Enable legacy Browser Support – See How to set the Forms.LegacyBrowserSupport application setting to "true" in the site's web.config below for further details on this option.
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5. Running the K2 Forms Viewer web part in a site on SharePoint 2013 in SharePoint 2010 mode where the master page document mode is set to IE=8 by default. Broken in 4.6.9 • Use a later version of Internet Explorer.
• Use a different browser.
• Prevent K2 smartforms from running in Compatibility mode or Enterprise mode in Internet Explorer by using Edge Mode – see KB001704 on how to configure Edge Mode.
• Get the available cold fix from Regional support.
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From 4.6.10 • Use a later version of Internet Explorer.
• Use a different browser.
• Prevent K2 smartforms from running in Compatibility mode or Enterprise mode in Internet Explorer by using Edge Mode – see KB001704 on how to configure Edge Mode.
• Enable legacy Browser Support – See How to set the Forms.LegacyBrowserSupport application setting to "true" in the site's web.config below for further details on this option.
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How to set the Forms.LegacyBrowserSupport application setting to "true" in the site's web.config

The Forms.LegacyBrowserSupport application setting has been introduced to allow for instances where you might have some dependencies on IE8 or integration with SharePoint 2010 which requires even later Internet Explorer versions to run in IE8 compatibility mode. The Legacy Browser support mode can be enabled for the SmartForm’s runtime site by setting the "Forms.LegacyBrowserSupport" application setting to true in the site's web.config. When this setting is enabled the Unsupported Browser message that would normally block the user will not be displayed, as older, deprecated JavaScript libraries are delivered to the browser instead of the newer and faster libraries which are not supported by IE8. 

Warning: Certain SmartForm controls have a dependency on the newer libraries. This resolution may result in errors or issues with Forms if the Forms.LegacyBrowserSupport is enabled -- some browser APIs and features may not be available. The controls in Control Pack 4.6.9 are an example of controls dependent on the newer libraries.

Debugging a Form: To assist in debugging these Forms, as the mode of the browser is not apparent when Forms.LegacyBrowserSupport is enabled, a message is logged to the browser console indicating that the browser or its compatibility mode is not supported. Run the Form or View in debug mode (?_debug=1). The message will be accessible in the browser console or the SmartForms Logging Dialog as shown below:

Debug Message: The attached page targets IE document mode 8 (or earlier), or is running in IE8 (or earlier). Some APIs and features may not be available. Errors are to be expected. See the K2 smartforms Compatibility Matrix (http://k2.com/help/K2forSP/WebPart/MoreInfo) for more.

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Warning: The Forms.LegacyBrowserSupport setting should not be used in instances where a better workaround of using a later version of Internet Explorer or using a different browser can be employed in your environment. Using this setting is not a best practice nor is it meant as a long-term solution.