I am talking about any scheme of how to do things that is intended to provide benefit. These all start with "wouldn't it be better if X, because Y" and then a plan is made of how to bring this about.
Well, this plan is inevitably imperfect, so it is either that you don't get all of X, or the reasons Y were not correctly understood or accounted for.
Then, there are always some extra drawbacks that creep in that negate some of the benefits. Usually these drawbacks are very subtle, and they can be hard to notice because they are not things that the plan was trying to address.
In the end, usually the net result is negative: the scheme causes more damage than it provides in benefit. But usually it takes a long time to understand this clearly, because the drawbacks can be subtle (but sometimes they aren't, for example, in TDD, how much extra code you are writing all the time).