Where does this IntegrityError comes from on the Django ORM on MySQL?

Question

I have a Django 1.5 site running a MySQL DB on Cent OS, and one of the views that used to worked suddenly started to fail on this snippet :

ipdb> Video.objects.create(owner=usr, category=category, status="processing", title=request.POST.get('title'))
*** IntegrityError: (1452, 'Cannot add or update a child row: a foreign key constraint fails (`project`.`videos_manager_video`, CONSTRAINT `category_id_refs_id_26102a51` FOREIGN KEY (`category_id`) REFERENCES `videos_manager_category (`id`))')

The code hasn't changed, the data base hasn't changed and we ran mysqlcheck (it said all tables were OK), but this still explodes.

Of course we checked category, which is a proper category object, with proper id. Doing this:

Category.objects.get(id=cated.id) 

returns the same categ.

We purged the cache, juste in case, but still a no go.

What could cause this ? What would solve this ?