Here’s a simple scenario of how it can go wrong in the 139, even with all the bells and whistles:
Fail to set a altitude (perhaps even have a minus value in...). Take off; establish a safe climb out profile; select ALTA; fail to notice that your safe profile has shifted to a descent... next call “150ft”, assuming you got higher than that; fail to react right promptly. Easy done, esp if you hit the upper modes early.