I have been using the YouTube IFrame Player API without problem for months and started to see this exception recently when the player loads:
Uncaught ReferenceError: y
This is a Google bug. It also happens on Youtube's developer page. https://developers.google.com/youtube/youtube_subscribe_button
The following bugs tracks this issue.
https://code.google.com/p/gdata-issues/issues/detail?id=8569 (Still active -as of Jan 5h 2017)
Please up vote on this bug using the links below so that it gets their attention. They have been sitting on it since Sep 9, 2016!!.