Trap streets.

Cartography Cabinet Magazine gets lost in trap streets:

"Anecdotally, many cartographers relate the stories of deliberate errors introduced into maps to prove their provenance, from kinks in rivers and the addition of small buildings to exaggerated curves in roads and the systematic alteration of minor digits in geographic coordinates; nothing that would put the user off-track, but enough to make a map’s actual origin certain. Most of the time, such tricks are indistinguishable from errata, but across enough sheets or data points they form a pattern, and organizations such as the Ordnance Survey employ teams of experts to ferret them out in their competitors’ products.
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There have been occasions when I've looked for a street, not found it, convinced myself that it didn't exist in the first place, then found it on the map I should have remembered to take with me in the first place.