Sometimes errors get "stuck" in the kotlin notebook and show up in the sidebar even though it has been fixed in the code (hovering over the actual code does not show any error). I don't know how it happened, and restarting IDE fixed it. Is this a known bug?

Thanks for mentions it out. I filed an issue for this https://youtrack.jetbrains.com/issue/KTNB-434/Errors-might-be-shown-in-status-bar-even-though-code-is-updated