I've been running Mountain Lion since it first came out in August (on a new MBP Retina), and I've just started having these problems in the past couple of weeks. It's with a Google calendar that my spouse keeps that I subscribe to:
username@gmail is spouse's gmail account. I've tried refreshes, reboots, remove-and-reinstall etc. ad nauseum, but nothing helps. Apple says it's Google's fault. Any suggestions, please?