Lesson learned after hours of debugging:

never never never ever use core data objects _after_you saved the context (without explicit retaining them)

They don’t even turn to Zombies. The App just dies hard. something like SIGSEGV — SEGV_ACCERR.
And the stack trace wasn’t helpful at all. F**k you, NXHashRemove.

My code looked somthing like this:

to fix the crashes, just move the save to bottom.

User gets invalidated after saving. Another way to fix it is to manually retain the object, save, and then release it.

Related posts:

1. Multithreading with Core Data
2. Core Data Notes from iPhone Tech Talk
3. CoreData and mergeChangesFromContextDidSaveNotification