This is very hard to reproduce. Some sort of race condition in Geeqie's directory traversal/file loading.
ERROR:filedata.c:693:file_data_unref_debug: assertion failed: (fd->magick == 0x12345678) (SIGABRT)
If one deletes images from the current directory with a tool external to Geeqie, Geeqie refreshes the directory contents and sometimes (!) crashes.
A fresh bug report and detailed backtrace for Geeqie 1.0: