The law of unintended consequences states that any purposeful action will produce at least one unintended consequences. This is not a scientific law but I have a hard time finding actions that do not have a unintended consequence.
Robert K. Merton listed five possible causes of unintented consequences:
- Ignorance (It is impossible to anticipate everything, thereby leading to incomplete analysis)
- Error (Incorrect analysis of the problem or following habits that worked in the past but may not apply to the current situation)
- Immediate interest, which may override long-term interests
- Basic values may require or prohibit certain actions even if the long-term result might be unfavorable (these long-term consequences may eventually cause changes in basic values)
- Self-defeating prophecy (Fear of some consequence drives people to find solutions before the problem occurs, thus the non-occurrence of the problem is unanticipated)
An example:
In 1990, the State of Victoria (Australia) made safety helmets mandatory for all bicycle riders. Together with a reduction in the absolute number of head injuries, there was also an unexpected reduction in the number of juvenile cyclists. Research by Vulcan et al. found that the reduction in juvenile cyclists was because the youths considered wearing a bicycle helmet unfashionable.
Try to think about the actions you have executed in the last 24 hours and what unintended consequence(s) you (could) have created 
