It is a design flaw in 2010
It is a design flaw in 2010-2012 engines. 2013 has revised head gasket, 2014 revised piston design, 2015 revised piston and piston rings design. So, overall 3rd generation Prius has wide range of reliability. 2013-2015 is way more reliable than the previous year. Unfortunately, that's why we usually want to avoid 1st year of any generation. Fortunately, the 4th generation has much less issues since it leaned from 3rd generation flaws. Only very minor problems in 2016 Prius.