We sometimes see customers running into problems when using the "Sign in as Different User" feature in SharePoint.
Common problems are
- Documents opened in external applications are saved with an unexpected user account because the external application runs under the windows account logged in to the machine and not the account you have chosen in SharePoint
- Page content shows information from the previous user due to cached information
- Session variables contain information from previous user as the cookies are not cleared
The Sign in as Different User functionality is not meant to be used as a security feature!
This feature allows users which have more than one AD account to quickly login with a different account to SharePoint - but it does not guarantee that no artefacts from the previous user remain!
If you have to guarantee that one user cannot see data from a different user you have to logoff/logon in Windows. Don't use the Sign in as a different user.