Category: Errors preserved in perpetuity

The list below is believed complete, though membership – like everything here – is subject to revision.

The dol – A real unit of pain. One dol was two just-noticeable differences of heat, the scale ran to ten and a half, and in 1947 a woman in labour reached the top.
The Incorrigible Register – The 1847 method of record in which no entry can be altered, no authority adjudicates, and disagreements are settled by counting copies.
The Mountains of Kong – A range invented in 1798 to rescue a theory about the Niger. It crossed West Africa on forty maps for ninety-four years. Nobody had ever seen it.
Mountweazel – A false entry planted in a reference work to catch plagiarists. Named after one of its own instances, and it proves the one thing that is not an offence.
The myth of the flat Earth – That medieval Europe thought the world flat is a Victorian invention, assembled from a novelist's fiction and put to work in the quarrel over evolution.
Null Island – 0°N 0°E, where there is no island. It was named, and given a square metre of land, so that data which failed to locate would land somewhere visible.
30 February 1712 – The only 30 February known to have occurred. Sweden converted to the Gregorian calendar gradually, lost track during a war, and got out by adding a day.
The 21 grams experiment – A bed on beam scales and six dying patients. The number everyone knows comes from one of them, and reached print only because a newspaper got there first.