I can reliably reproduce the behaviour (the steps are sufficient, but not all may be necessary):
1. Grab a screenshot
2. Crop to a small part of the image
3. File->Save as... and save to a png with default settings
4. Ctrl-Z do undo crop
5. Crop to a different part of the image
6. File->Save as... and save to a png with default settings
I can reliably reproduce the behaviour (the steps are sufficient, but not all may be necessary):
1. Grab a screenshot
2. Crop to a small part of the image
3. File->Save as... and save to a png with default settings
4. Ctrl-Z do undo crop
5. Crop to a different part of the image
6. File->Save as... and save to a png with default settings
The second file will be saved, then gimp crashes.
I attach a gdb backtrace with debug symbols.