If you try to open a link to slack in the browser (e.g. from hangouts), slack tries to redirect you to the desktop app. However, if the desktop app is not running, slack on desktop just starts and does not direct you to whatever the link was about. You have to try it again once it's started to have it work.
Maybe it'd be just better to open the link in the browser if I'm already logged in there. Or fix this.