“… they stumble that run fast.”
This applies to Romeo and Juliet’s in the way that they run head first into a relationship. They take no time to slow down and get to know each other, instead within the first 24 hours of meeting they decide to get married. This, by no means, is not going slow. In their minds it may seem like the only and best option but it could very well turn around and be a dreadful choice. The dramatic turn of events represents a stumble. A stumble that occurred as they were running so fast into unknown grounds.