I scanned an old wedding album. When I was done I had over 200 images, some from an Epson scanner but most from running prints through a Fujitsu ScanSnap document scanner. The ScanSnap produces high contrast over-saturated scans, but with some post-processing they’re really not bad — and it’s fast.
All seemed well. I processed them in an Aperture Library, exported version JPEGs and imported those into my main Library.
Then I tried “Write IPTC metadata to Originals” and Aperture crashed hard. Again and again and again — with various timing and screwy error messages. It’s all documented in a long Apple Discussions where “léonie” (level 10!) and I worked through this.
Something in the IPTC metadata process was causing Aperture to write to random memory and eventually crash itself. Database repair would find lost _temp files (duplicates created during IPTC process) that seemed to cause their own problems. In particular they triggered spurious error messages about “is locked or you do not have permission to modify it” (Aperture gives this message when a file is kind of half-there). At one point I discovered that processing a shared iCloud photo stream that contained these images would crash Aperture!
There was a fix, but it’s unsatisfying. I had to change every possible metadata setting. Advance time on all images 1 second. Remove all Location data. Use "add metadata from: EXIF and iPhoto" (not sure what that does!) and fill out all the possible fields.
After doing all of this, basically rewriting every metadata field Aperture deals with, I could “Write IPTC metadata to Originals” repeatedly without a problem.