“The Unintended Consequences of Everyday Actions: A Reflection on Externalities”

An externality occurs when either a cost or benefit is imposed or conferred on a third party that is not directly related to a transaction.  Often times, as we go about our day to day lives we give little thought to the externalities we might be creating.      
Provide an example of something you have done recently that has created either a positive or negative externality, and whether or not you considered this externality when you decided to undertake that activity.   
No citations, please