Incremental build ignores changes within a function til Clean is performed

### Failing incremental builds

I don't have too many specifics on how to reproduce, but it was very strange when I realized what was happening. Definitely not a huge problem, or even a daily one, but one that's out there and maybe deserves some answers. If I run into it again let me know what logs or files to archive (besides a msbuild diagnostic output).

#### The issue
I was debugging a net4.6.1 console app out of VS 2017, making changes, pressing F5. The console app involved parallel loops and the IO class, but no nugets or anything more than "basic scripting utilities".

Then I noticed I had commented out a particular line, but in my manual testing it was insane! The line was still being executed each time I pressed F5. I did a double take, and assured myself that code was no where else in the project but that one line which had now been commented out. I tried commenting out nearby lines (in the same while loop) and it seemed like everything in that loop was being compiled without the comments. Weird.

I only verified this out the IDE, I am assuming the command line would also require a clean.

#### What I did
Just went with my gut at that point, did a clean, a fresh build and the problem was solved. Hasn't come back since, that was the first time I ran into it in months.

Any tips for next time are appreciated, on how to capture logs and create a bug-report