Through yet more research, the developer finally discovered that the problem was related to a shift in focus from one object to the other. In this case, it was a competition between two objects for system resources, a problem that would never be manifested when either object was tested individually. As we have already seen, objects exist as part of a larger system, and problems can arise either from the way the system interacts with the object or the way the objects interact with each other. Objects aren't like Lego blocks that simply snap together, they are more like bricks that need mortar between them. This mortar includes both application and system code that interacts with the object.
Simple things can creep in, such as incompatibilities between versions of shared system components.
I have seen one object refer to a dynamic library that another also needed, but they each expected a different version and neither was compatible nor able to peacefully co-exist. Where to Draw the Line Testing is all about assessing risk and setting your priorities accordingly. It is highly unlikely you will have the time or resources to test your application as thoroughly as you will want or need to, so it makes sense to look for ways to reduce your efforts. However, the idea that you don't need to test components just because they were developed by others is simply not true. What it does mean is that you need to test your particular implementation of it to be sure it does what you need and in the way you expect.