"You can't change just one thing" is an engineering principle that has a wide application beyond engineering. Applied to planning, it is a warning to consider secondary effects during the analysis phase.
Primary effects are intentional outcomes that come directly from the success of the plan. We usually don't have much of a problem identifying them, since they are why we have a plan in the first place.
Paraphrased, the question I've been asked by a lot of clients is, "How do we get there from here?" I always add, "without breaking something else along the way." I make it clear from the beginning that I'll be looking for impacts beyond the immediate goal. The client has never objected.
Secondary effects have a lot of names, 'side-effect', 'unintended consequence', 'collateral damage', 'byproduct', etc. (almost never 'mistake') They differ from primary effects mostly by having a negative impact. That makes them attractive to ignore. "What else happens if the plan works?" is not a question you hear very often, but it needs to be asked, and the answers need to be dealt with in the plan.
The first place to look is in the immediate surroundings:
- In the case of software, what will it do to the load on the computer, the network, storage, etc.
- Information systems in general either expose or hide information in new ways. Will we be putting sensitive information at risk?
- New business processes affect workflows. Are we fixing one problem to create another?
These are the easy ones to find because they are an immediate effect of the plan. Others may not be so easily found.
Something most marketers know is that introducing a product changes the market. The next place to look is at the impact on the victims--the people who will have to live with the results, and how their behaviour is likely to change.
- Are we changing incentives? New incentives usually mean new behaviour--not necessarily the behaviour we want. Sales managers fiddling with commission plans often learn that the hard way.
- Are we introducing a speed bump that people will work around?
- Are we making it harder for our clients to do business with us?
I once worked with a client on a program to produce a major improvement in the quality of Help Desk service. One of the impacts of success would be an increase in support calls; users were living with problems, working around them, and asking coworkers for help, all to avoid the indignity of calling the Help Desk. If the Help Desk's problems were fixed, users would be more likely to call them for help; the workload would go up as the service improved.
The point is that secondary effects can be counter-intuitive, so we need to look doubly hard for them.