I wanted to use magnet to change clothes, hands and head very quickly during a show… This is a bad idea since it disturbs the dynamixel bus and checksum errors appear when the magnet move.
Prefer velcro or others.
I have lost enough time because of this, that is why I report it here. 
Thanks for this tips! This is due to magnetics encoders of dynamixel motors?
No, I did put the magnets at the link between the head and the neck where there are AX-18(with classical encoder). The checksum error appeared when the head moved. I thought it was because of AX-18 but after changing them, there was no change. When I removed the magnet, all was OK.
… unfortunately, I had also an issue with an AX-18 servo (I changed the bad one) But I do not know if the servo was broken because of the magnet.
You still using magnet or not?
What kind of magnet do you use? There are really strong?
I used very powerful magnets yes but now I removed them.
I really do not understand what happens… I am investigating.
The error occurs but not each time I move the head, this time, but every 20 min (approximately)
I guess this is because I use only 1 USB2AX, the command sequence is too long and a checksum error can happen more often. I will definitely but the 2 USB2AX to see if it happens.
Did it happen for anyone this “checksum error” ?
OK, now I know all about my issue 
I plugged the two USB2AX devices, and there is no way to have checksum errors ! Even if I take a powerful magnet and I frenetically shake it on Poppy body (magnet massage…)
So, there is no issue about magnets.
And using 2 USB2AX is necessary to improve time response BUT ALSO dynamixel bus robustness.
I changed the title of the topic.
Great!
Mystery no more…