As I understood it Google updated Hangouts to use a PNaCl so that the needed code would be downloaded automatically instead of requiring users to install a plug

As I understood it Google updated Hangouts to use a PNaCl so that the needed code would be downloaded automatically instead of requiring users to install a plugin. Browsers that don't support PNaCl should have continued working with the legacy plugin. I'm not particularly familiar with the transition but it sounds like the unintentionally broke the older implementation. The Google+ team in particularly seems to iterate quickly so it's not particularly surprising things will break from time to time.