Ok in scenario A we do X, Y, Z, and in scenario B we'll do this other stuff mostly the same but a little different. That will <puts comments> go here ...

Ok let's make A work....

-time / days passes-

Ok we're done and it is ready to go.

-later-

Someone: "Why doesn't scenario B work?"

Me: LOL some idiot .... <copy paste, fiddle, copy paste, fiddle> there we go I fixed it!!!