I've got a C# form, with various controls on it. The form controls an ongoing process, and there are many, many aspects that need to be right for the program to run correctly.

Each part can be unit tested (for instance, loading some coefficients, drawing some diagnostics) but I often run into problems that are best described with an example:

"If I click here, then here, then change this, then re-open the form, then click here, it crashes or produces an error"

I've tried my best to use common code organisational ideas (inheritance, DRY, separation of concerns) but there never seems to be a way to test every single path, and inevitably, a form with several controls will have a huge number of ways to execute.

What can I read (preferably online) that addresses this kind of issue, and is there a (non-generic) term for it. This isn't a specific problem I'm having, but one that creeps up on me, especially with WinForms.