Can a plugin cause permanent damage?

Is it possible for a plugin to make permanent changes to a WordPress site? i.e. if I install and activate a plugin, test it out and find it’s no good for me, and deactivate it, is there any way it could leave the WordPress site in a different state than it was before I activated it?

e.g. files in the Media Library altered/added/deleted

e.g. other changes to code or database (outside of the plugin’s own data/code)

In my case, I’m considering installing a plugin that handles lazy loading of images. These work by replacing thumbnail images with a placeholder image and handling loading of actual thumbnails as they become visible on the page. I’m worried that they might permanently replace an image in the media library as part of their setup process and that these would be left broken if I deactivated the plugin. Is that possible? (Especially as every lazy load plugin I’ve looked at has some 1-star reviews saying it doesn’t work in certain situations.)

Yes, it’s absolutely possible for a plugin to make permanent changes to a site. The plugin could add to or modify the contents of the database and it could remove files (typically this would apply to media).

It’s not uncommon for plugins to leave behind orphaned data such as options in the database. This is fairly harmless, but sometimes plugins go overboard by adding many additional rows for options.

To be on the safe side, you should read over the plugin’s code and test the plugin on a staging site.

Hopefully the plugin you are describing is using a filter to modify the output of the post thumbnail on the fly, which would not actually modify the contents of the database, but to be sure the plugin’s code would need to be audited.