I was studying SCM/Bug tracker integration when I found that it is not only possible to close and modify issues from the commit log, but it is also possible to create them.
But since the commit log is for code changes, I don't see why anyone would modify the code to create an issue. Please cite one scenario where this is justifiable.
One scenario might be where you're committing the fix for a particular issue, but you know the issue you've resolved is a short term fix for the problem.
In this case it might be necessary to open another issue in light of the short term fix so that you know you need to provide a long term fix somewhere down the road.