For the last few days, clicking on a folder in the left column to change folders will cause Safari to display the spinning rainbow wheel of death until it is quit. I have tried this on 3 separate macs all running OS X 10.9 and the behavior is consistent across them.
Note that Safari doesn’t crash completely. Downloads will continue downloading and you don’t have to Force Quit the app, but NewsBlur is completely hung and unusable until Safari is Quit and restarted.