[magicalhippo]

Good question. Obviously a profiler or similar that can capture the details when it happens helps, as you note.

If you can reproduce the issue then what I tend to do is to include the unique id column from each joined table (we try to avoid natural keys).

If it doesn't have a unique id column I replace the join with a subquery that includes row_number(), so I can se which one that doesn't repeat.

But without being able to replicate, I don't know of any better way than just studying the ON conditions carefully.