SQLite occasionally fails to create :memory: database

In our unit testing suite, we create and destroy a large number of SQLite databases that use the path of ":memory:". Occasionally, and only when running on the iOS simulator, the creation of those databases fails with the rather enigmatic message:

Database ":memory:": unable to open database file

99% of the time, these requests succeed. (Subsequent tests within the same test run typically succeed after this failure occurs.) But when you're using this in an automated build-acceptance test, you want 100%.

We've instrumented for memory consumption (it's within normal limits) and disk-space availability (20GB+ available).

Any ideas?