There certainly seems to be some airfields with threshold crossing altitudes below 50'. As far as I can determine according to Annex 14, the maximum slope in the first 25% of the runway (essentially the touchdown zone) is 0.8%. This could mean a threshold height of as little as 30' for a 3 degree slope, which seems a bit extreme. This also makes the maximum crossing alt 66', so disregard the 1% example!!
However, from what I can determine the design of a non-precision approach, the glidepath is worked back from 50' above the threshold, so threshold crossing heights other than 50' seem to apply only to precision approaches.