A friend recently had to spend about 30% of the cost of his Trex 600 Nitro heli in two crashes, because of a simple mistake.  He had put on Futaba servo arms on Hitec servos    It was not obvious, so initially, the crashes remained unexplained  

After some time in this hobby, people tend to pick up servos (and other equipment) from various brands.  After some years, most people end up with a box of parts, where similar parts from different brands are all thrown in together.  

Just be aware that the Futaba arms are different from Hitech and Hitech is different from JR (and this probably applies to other brands too).  Do not risk your aircraft by substituting what is available.  Use the right servo arm appropriate to the brand of servo you are using.

This is why model manufacturers who make custom arms for their aircraft (to get lengths correct) make them for all 3 servo brands. For example, the custom servo arms for Trex 600/700 helis from Align come in all 3 flavors : Futaba, Hitec and
JR

And as much as possible, always lock the arm in place on the servo with a servo arm screw.

Thanks for this reminder Anwar ,
I have conveniently put all the extra horns in  box thinking that is neat , now that you have said it I don't know which horn is for  which servo so I guess this is quite a common mistake  and as you buy more and more servos you will accumulate  more and more extra horns without knowing the servo they were meant for 

Now the only option is to  actually try all horns each time I need one for a servo

Sai

The bad thing is that it was very misleading !  It felt like a good fit and the swash etc were moving fine.  Initial hovering tests also went fine. MY friend changed the servos, thinking they are the ones that are bad

Only after crashing couple of times it was noticed accidentally that we could move/rotate the arms manually with a little bit more force (while the servos where turned on).

That was surely an informative post. Till now i have had only Futaba servos, but slowly adding the Hitec ones to the mix, so will be careful !
Thanks Anwar.

The difference is very marginal.
The 3 companies Futaba, Hitec and JR  have a different number of teeth on the servo cog onto which the arm is screwed.

Something like 23, 24 and 27 teeth. I remember reading somewhere.
So if forced in the plastic arms will "fit", but will not perform precisely in a helicoptor - like scenerio.
Avijit

I have done forcing a futaba arm on a JR mini servo. It was a tight fit but I was a bit paranoid and dripped down a drop of CA. I never had an issue with it till the last day I flew the model.

But then I could be lucky and I do not advise anyone to follow it.

-Ismail

IMHO, it is not at all a question of precision. It is all about RISK.