Worst fault or, if you prefer, trait most likely to cause problems.
I think, for me, it may be lack of self-confidence and mistrust of my inner voice. I have an INTJ friend who once said, "I always listen to myself." Not boasting, just objectively stating a fact, and puzzled that I don't do the same. Why wouldn't anyone? For my part I envy that quality so much--I so wish I had somehow discovered the seed of that quality when young and nurtured it before it was too late.
I'm the opposite. I tend to mistrust myself, and especially when I was young, I either ignored my inner voice or did the opposite of what it was saying when making decisions--including major life decisions. The result has been a lot of wasted potential, and a *lot* of misery for myself and those around me.
Of course, that's my own personal huge character flaw, but I suspect that being an INFP made me susceptible to it.