When you install and register Visual Studio Tools for Office (VSTO) application-level Add-Ins for Microsoft Office, there's an optional registry entry named Warmup, which can be set.
The Registry Entries for Application-Level Add-Ins topic on MSDN explains as follows:
Optional. A value that indicates that the .NET Framework and Visual Studio Tools for Office runtime load before an add-in loads and reduce the perceived time to load an add-in. Set the Warmup entry to 1 and use this in conjunction with LoadBehavior entry to reduce the load time for Outlook 2010 add-ins that are deployed by using Windows Installer (.msi). This registry key cannot be set by using ClickOnce.
If this entry is enabled (set to 0x1), the following event is logged in the Application log when the application starts and loads the add-in:
Event ID: 4096
Customization URI: file:///C:/Program Files (x86)/Common Files/Microsoft Shared/VSTA/Pipeline.v10.0/PipelineSegments.store
Exception: Exception reading manifest from file:///C:/Program%20Files%20(x86)/Common%20Files/Microsoft%20Shared/VSTA/Pipeline.v10.0/PipelineSegments.store: the manifest may not be valid or the file could not be opened.
This event can be safely ignored and does not affect the functionality of either the application or the add-in.