
This happens when trying to unlearn light armour training.

In the same way, this happened when trying to unlearn forceshield.

And those can't be really strong but if I can stack them forever then lure mobs to them... I used multiply until I couldn't, then let go of the possession and did it again.
And lastly my possessor got disarm trap when he found an thief escort in Trollmire. Just by finding him, not escorting him to the exit and taking a reward. Did not happen with other escorts so far, need to find a thief one again to see if it happens everytime. Also escorts count as summons? I tried to posses one and got a message saying I couldn't possess a summon.
Edit : it would seem you gain disarm trap at level 2 no matter what actually...
That's all for now I think.