I was reading the History of the Fall and Decline of the Roman Empire (Edward Gibbon, 1776) when I encountered the phrase 'prudent vigour'. Prudent means carefulness and showing forethought while vigour means energy and enthusiasm. Aren't these two things incompatible? And hence the phrase is an oxymoron.
The experience of Augustus added weight to these salutary reflections, and effectually convinced him that, by the prudent vigor of his counsels, it would be easy to secure every concession which the safety or the dignity of Rome might require from the most formidable barbarians.