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Not splitting stuff up into multiple applications. It's not so much about reusability as it is about having a dozen models, and over 100 views in one app, it's damned unreadable. Plus I like to be able to scan my urls.py file easily to see where a URL points, when I have 100 URLs that gets harder.
I've experienced a worst practice : use something else that the default id as primary key of model.
It looked as a good idea but it caused some problems in the administration web site and it was difficult to restore the id as primary key on an existing database.
I think that there is no specific case that are specific enough to cause these problems. I recommend to keep the id of your model as it is by default.
See What is the best approach to change primary keys in an existing Django app? for details