I'm having a hard time deciding and finding two actions/decisions that are questionable in all of act one.
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The quarrel between the Montagues and the Capulets is questionable - they have been warned not to behave in such disgraceful fashion.
A second one might be when Romeo and Benvolio decide to go to the Capulet ball. Had they not done this, Romeo would never have seen Juliet and realized how beautiful she was compared to Rosaline.