Steps to reproduce:
- go to Packages > My packages
- check some packages (one or more)
- click a package name to see its details
- when on the package page, right-click and select Back in the context menu
Now the line above the buttons reads "Selected packages:" (blank), and buttons are grey out. But the packages are still checked, and if you uncheck them, you get a negative number of selected packages (and this activates the buttons :) ).