My best was 5000:1 by updating the outdated SQLite in one of the companies' products.

I found that we were hit by a bug that was fixed 6 years before I discovered it (https://sqlite.org/src/info/6f2222d550f5b0ee7ed). Sqlite's query planner assumed that a field with a not null constraint can never be null, which isn't the case for the right hand table in a left join.