As a WordPress Multi-Site administrator, make sure a plugin works with multi-site before installing it. Plugin authors should state if the plugin works with multi-site.
Testing is best done on a separate installation of WordPress (the one good reason for having multiple WordPress installations), so if there is a problem it doesn’t affect your production site.
Plugin Authors: If you have tested the plugin with multi-site, then claim the feature in your marketing. I would not use a plugin that doesn’t say it works on multi-site, since even though I am technically qualified to do that testing it is much too much work to have to do.
Does the plugin always request the table prefix from WP? (One immediate clue is a plugin that uses ‘wp_’, the default, even though that isn’t what you used when installing WordPress; then WordPress appends the site ID to that prefix for all user data.)
Does the plugin always request the folder locations from WP? In multi-site mode, each site gets a unique file upload location.
Does each site of the multi-site get its own set of plugin tables, for plugin options and for user data?
When a plugin is activated on an existing site, are tables checked for, and created if needed? (Or do thousands of error messages about missing tables clutter the error log? Yuck!)
When the plugin is network activated, and a new site is created, does the plugin activation work? Is there a reminder in administration panels, to configure the plugin for the site?
When a site is deleted, does the plugin deactivation work?
If the plugin creates file (for example, a podcast creating plugin would have the podcast episodes listed in a .xml file), does each site get a separate result file, or separate collection of files?
Improper multi-site use would assume the table prefix, or assume the folder locations, or use one set of plugin tables for all users, or use one folder for all user’s media files, or allow unauthorized people to include in their post or results some information from other users, or allow unauthorized people to modify someone else’s tables or files.
Plugin Author: State in the installation instructions or FAQ whether the plugin is safe to use in a multi-site installation. State whether the plugin can be network activated or must be activated on each site. State in the change log when multi-site features are added or fixed.