Friendly fire, isn't. But it seems to happen in every armed conflict, regardless of who is involved and how well they're trained.
It seems that when air support is not under direct control of the ground forces, or at least not in direct communication, these incidents are far more common than when it is. The further removed the guy in the air is from the guy on the ground, the more likely things will go bad. Even moreso when they are not even from the same country, regardless of being separated by a common language.
It's always a shame when it happens, and it makes a combat loss even more painful.