My biggest personality fault is a reason why I don't trust a lot of people and vice versa.
I'm fairly introverted at times and reticent, I don't tell people my problems because it's none of their business.
Also if someone hurts me in anyway ( whether it be a relative, friend or a complete stranger ) I can unfortunately turn quite vindictive and will wait quite a long time to get back at them ( days, weeks, months, years; ect ect. ), even at an inappropriate time. Which ends up leaving a lot of people confused as to why I did it.
Like an elephant, I'll never forget.