For instance, the unexpected benefit is where a positive result is also received along with the intended result of the action. This positive result could be considered as good luck on the originator's part. Such as aspirin being developed as a mild pain reliever. Who knew in advance it would also turn out to lower the risk of heart attacks?
The second category can be an unexpected drawback, where the intended result is achieved, but it is accompanied by a downside to the action. An example would be where the use of antibiotics allow a person to combat germs, however some germs have then afterwards grown stronger and became antibiotic resistant.
And lastly, there can be a perverse effect which is a result contrary to the intended effect. In British colonial India, the government had concerns about too many cobras in Delhi, so they offered a bounty for dead cobras. This policy worked for the reduction of those reptiles until most of the snakes were gone. At that point, to continue getting bounty money from the government, people began breeding their own cobras that they could kill and turn in for payment. Realizing what was going on, the government cancelled the bounty program. Now left with quantities of worthless snakes, the people breeding cobras turned them loose. In the end, there were more cobras than there had been in the beginning.
However, since humans tend to believe they can fully control the world about them, it appears that The Law of Unintended Consequences will continue to survive.
Which brings us to storytelling.
How often in your plotting, either consciously or subconsciously, for a new story, do you place your characters in positions where a decision for a course of action, with all good intentions, then produces an unexpected drawback or perverse effect for those characters to deal with? It's a good way to provide conflict between characters, and if you're not already taking advantage of these two ploys in your manuscript, you might want to consider how you could use them to increase the story tension. As the bikers say in Sturgis, "Crap occurs." (I kinda cleaned that up in case tender young minds were reading this.)
In the end, whether your characters have good intentions on the road to hell or face similar circumstances to The Road to Perdition, they're bound to fall victim to The Law of Unintended Consequences and your readers will find themselves involved in a page turner to find out what happens next. Go forth and see if it works for you.