Submitted by Omik, also found by gpersoon
The implementation of the transfer function in nTokenAction.sol is different from the usual erc20 token transfer function.
 This happens because it counts the incentive that the user gets, but with a self-transfer,  it can lead to unlimited mint. In L278, it makes the amount negative, but in L279, it returns the value to an amount that is not negative. So, in the L281-282, it finalizes a positive value, only because the negative value is changed to the positive value.
 You can interact with this transfer function through nTokenERC20Proxy.sol.
Recommend adding (sender != recipient).
