Good Intentions vs Bad Consequences
When Tybalt enters the Capulets household to fight Romeo, Mercutio has good intentions of standing up for Romeo. Romeo however, does not want Tybalt and Mercutio to fight because Tybalt is Juliet's cousin which Romeo is madly in love with. In result of Romeo stepping between Tybalt and Mercutio, Mercutio is very badly hurt and eventually dies. This would be the bad consequence.
Another good intention that resulted in a bad consequence would be when Romeo & Juliet decided to get married and keep their relationship a secret from both of their families. In their opinion they were doing the right thing by limiting conflict between the two families, but what they don't is that it will cause a very large amount of conflict. The bad consequences that come out of the good intentions would be the death of Tybalt & Mercutio, and then essentially themselves.
A real life situation of good intentions and bad consequences would be for example: Telling your parents that you're going to a friends house for a sleepover but those are not your real plans. You end up going to a huge party that your parents don't know about. The consequences from your good intentions of having a fun time with your friends could be, getting grounded, losing your parents trust, getting your privileges taken away, etc.