Not sure if this is a WordPress issue or one with the theme (Tarski), but I've not had it happen before.
I decided to put the Tarski theme on a site that didn't use it before. When I switched to the theme, I tried to set up the widgets and move them around. Once I removed a few, they still behaved as if they had been placed in a different sidebar and because inactive so I couldn't select them.
Right now, I am using zero widgets. But the Pages/Archives/Links/Meta/Search/Recent Posts widgets are unable to be added to any sidebars.
I've tried clearing my browser's cache, logging out and logging back in, switching back and forth between themes, and I don't get what's going on. Is there a way to manually designate where you want your widgets to go?