Orchestral instruments don’t come more treacherous than the French horn, either for the musicians who play it, or, when the going gets rough, for the listeners who find themselves within earshot. Sometimes you wonder how the instrument found its way from the hunting lodge to the orchestra.

At the Mostly Mozart Festival in recent weeks, the intractable early version of the horn has made its way into the Rose Theater as a series of period-instrument bands from Germany, England and Italy performed music ranging from Italian Baroque choral works to Mozart opera. When these groups were at their best, a listener whose fondness for period instruments dates to the 1960s could reflect on how far the performance standard has risen since.

In those days period ensembles that sounded vigorous on disc often proved anemic in concert, and the instruments’ antique technology was regularly blamed for mediocre performances. Nowadays, the performances are more typically extroverted and expressive, and although period instruments, by definition, have not been modernized to make them easier to play, listeners are no longer asked to consider their difficulty when a performance goes awry.

Except, that is, when the horn notes crack and slither. The horn remains the wild card in period-instrument orchestras, and in modern ones too. And if you find yourself cringing when horn players falter badly  as I did on Aug. 5, when Concerto Italiano played three Vivaldi concertos with prominent horn parts  caveats about the instrument’s intransigence come quickly to mind.