Postgis reporting a point is 1216m away from itself using ST_Distance I have a table containing a geometry column in 4236 projection. I take the ST_X and ST_Y values from one row, and create a point from them. I then transform the point and the original geometry column value into 3857 projection to get a 2d distance between then using ST_DISTANCE. I expect the distance to be ~0. The answer I actually get it 1216.something metres.
Here is example code which creates two points, which should be identical, and takes the distance between them, giving the same ~1216m answer
select ST_Distance(
               ST_Transform(
                       ST_SetSRID(
                               ST_MakePoint(5.6100, 58.8900), 4236),
                       3857),
               ST_Transform(
                       ST_SetSRID(
                               ST_MakePoint(5.6100, 58.8900), 4326),
                       3857));

I'm presumably making some stupid mistake here (I am by no means a GIS expect), but what is it that I'm doing wrong?
The two points use different ellipsoids.
EPSG:4326 is WGS 84.
EPSG:4236 is Hu Tzu Shan 1950.