A helpful Bookcase user pointed out two bugs in version 0.6.5 to me in an email. In the Field Editor, if you change a title, click 'Apply', change another title, then click 'Apply, Bookcase crashes. That's an easy fix, and already in my development version of 0.6.6. For now, just don't click 'Apply' until after you make all the changes you want.
The second bug is a little more complex. If a field title is changed, then saving the currently visible columns in the view doesn't work right. That's a problem mostly, again, for translated versions of Bookcase, but also for anyone that renames some field. I'm currently refactoring some of the code I use for saving settings and using them when a file is opened, so I think I'll have this fixed, but not until the next big version, 0.7.