What problems do you have when debugging other people's code?

I am working on a debugging tool in a very small niche. I'd isolated the issues with debugging to 3 steps.

1.Know there is a bug
2.Find the bug
3.Fix the bug

Step number 1 can be solved with unit tests. I created a way to dynamically create them upon finishing a piece of code. Small , but useful.

For step 2 I created a flowchart of the running app so you can see the code around a function. All of the functions that called the current one and everything that function calls . So you can follow how this piece plays into the entire application.

Step 3 is hard because it is a little abstract. A bug is the code not doing what you would like it to do. So you would have to have a clear picture of what you want done. Then compare it to what you have. And you essentially just follow the code and steps line by line until one of them differs.

Any feedback on anything said here or the title would be appreciated. Let me know your best debugging tools and let me know if I'm missing anything.

Thanks!