After the customer reported being unable to load the classic editor we tested on a couple of instances.
Test steps:

1. Load workflow in browser
2. Go to diagram mode
3. Click the "Classic Workflow" link
4. Page renders improperly

This was tested only on an OnDemand instance of JIRA against IE8 and IE9 on XP and Win7. One test each, so it's possible local settings are interfering.