Yes that was 2011 but it scared the hell out of me. Since then all my company's project are hosted on Heroku and other Amazon technologies (Simpledb, Dynamo etc.). If I had to have a vendor lock in in exchange of faster development process, I'd prefer to get locked in with Amazon other than Google after that GAE pricing incident.
It wasn't easy to mitigate: re-writes were needed because of the private apis. We ended up doing a lot of optimization to prevent a pet project from costing us too much money, which before the price increase stayed well under the charging threshold. The pricing incident was the only thing on my mind about Google's cloud because we left Google since then. I'm sure things are different now, but then I got too many other things to do before taking a serious look at ditching aws for google.
Seeing as I can't reply to the comment in question... the price hike article you mention is in 2011. The price hike was a change of system, and everything went up because most people were using Python 2.5 which had no threading support. Python 2.7 had proper threading support, and unsurprisingly when that was enabled, everyone's cost dropped by three-quarters. Google dropped support for 2.5 two years ago.
If were really worried about costs, you'd convert your App Engine app to Go.