Partial position search on my chess database went through a stretch where every result came back from the 1890s. No date filter was set. It just did that.
Each position maps to a list of game ids, and I was slicing the first 4,000 off that list before ranking anything. Ids run roughly oldest to newest, so the pool being ranked was the oldest games in the corpus and nothing modern ever got considered.
It survived a while because it looked fine. Fast, and the games really did contain the position you asked for. You'd have to know there were 10.4 million games in there to notice yours only ever came from a few thousand.
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