I was a mistake.
( I ( ( was ( a mistake ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBD was) (NP (DT a) (NN mistake))) (. .)))
I wasn't supposed to be here.
( I ( ( ( was n't ) ( supposed ( to ( be here ) ) ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBD was) (RB n't) (VP (VBN supposed) (S (VP (TO to) (VP (VB be) (ADVP (RB here))))))) (. .)))