Is it common to end a Marie Curie IF fellowship early?
I don't know if it's common—one would require statistical evidence to answer that—but it certainly happens. A former colleague of mine finished a Marie Curie early (not a year, but perhaps 9 months) in order to take up a different position. In their case it was not even a permanent position, but another postdoc (a Veni grant in the Netherlands), although they obtained a permanent position not long after (and cut short their Veni to take it up).