HowStuffWorks has a good introduction on how air conditioning systems work. Whether it is a house or a car, the components are similar. I'm not going to go into too much detail as there is a ton of information available with a quick Google search.
The A/C system has a few main components: the evaporator, the condenser, and the compressor. There are other parts mentioned in the HowStuffWorks article, but let's focus on these three. Let's add a fourth component: these three are connected via hoses.
If you have a problem with your A/C, or you simply want to upgrade to a more energy-efficient unit, you will most likely work on one or more of these components, but not the hoses. Hoses are, at least in a modern house, tucked inside the walls and are hard to replace.
If you had a problem with cooling, you will most likely refill your system with refrigerant from the outside unit, or in the case of an automobile, on the valves along the hoses nearer to the compressor. If you wanted to upgrade your compressor and evaporator, you will be working on components outside the cabin of your car.
While the technician is replacing components or recharging with refrigerant, he may notice that there are leaks in your hoses, or the connections in between hoses. But this does not necessarily mean that the new components are defective, or that the refrigerant is bad. A different component is failing, not the ones he is currently working on.
Similarly, business applications, for example, one for Customer Relationship Management and one for Order Management, are connected by "hoses", in software terms, "Middleware", or more recently "Enterprise Application Integration" software.
If you are making changes on one or both applications, and expect messages to be passed between them, it is entirely possible that the "hose" is broken, and not necessarily that the applications you have changed are broken.
Of course, it is necessary to fix the "hose" in order to test that the messages with the new or updated information is being exchanged. But, if after a few messages have been successfully sent, and then it breaks, and no new changes were introduced to either application, then the "hose" has a problem, not the applications.
The point is that it is possible that the application changes are correct, and can be considered successful. The changes can be put into production, while the problem with the "hose" can be addressed separately. In the A/C world, the technician can still continue to replace the components, and deal with the hose issues separately.
