Installed Popular Widget on WP 3.2.1 blog that uses the Photog theme. Enabled pretty much the same settings as in the plugin page screenshot except for 5 posts and 7 days. Loaded first the About page and the widget showed garbled like this.
I then loaded some category pages, the front page and post pages and the about page again. All of them now showed a garbled footer and the Slick Contact Forms widget was displayed in the widget bar instead of the footer area where it should be, opened up instead of click-to-slide-open and with the fields visible that should not be not visible to users.
Most of the posts are embedded content for example from Youtube.
According to the description of this plugin it would be exactly what I need, so I would hope for this bug to be solved. As ArielK reported, a welcome addition would be ability to add a title to the widget and an explanation of what do all the widget options do, eg. Views vs. Visits.
Also, the FAQ and Description page link here on WordPress.org lead to a 404 error page.