For one product I worked on, its core was a custom ORM. The product had to work with an unknown database schema - adapt to customer's database - and provide fun

For one product I worked on, its core was a custom ORM. The product had to work with an unknown database schema - adapt to customer's database - and provide functionality similar to MS Access but specialized for pharma/chemical industry. There was no ORM flexible enough for this use case. Then, jOOQ came out and it could be usable with a lot of extending for the chemistry stuff. But at that point, we already had the custom ORM.