Basically every now and then you get a "fltk.prefs" in some directory on your system. It seems a bit random because the relevant code is only triggered in some places, and the user might not have permissions to create this file.
The bug is that when FLTK constructs its Fl_Preferences hierarchy it is marked as dirty even though nothing has changed. Then it is automatically written to disk in the destructor (which is called pretty much right away).
For our purposes we can probably neuter this entire functionality.