You're working fast, in the flow, and just wrapped up something great. Tests pass, time to commit.
Ugh, you weren't disciplined: you did two things at once and the other thing isn't finished. Even if it was finished, it doesn't belong in an isolated commit.
Partial commits to the rescue.
Start your commit, and in the commit dialog, de-select the files you want to omit. (Or click the root to de-select everything, then select the files that you want in the commit.)
That let's you do a partial commit with the not-ready files left out. But what if a file has some parts that are done and others that aren't?
Partial commit also applies to chunks in a file. In the commit dialog's diff viewer, un-check the parts of the file you want to not commit.
Once you have the subset of changes that you actually want to commit, proceed as usual.