The German magazine Uhu
reported that x-ray analysis of Thomas Gainborough's famous work The Blue Boy
(completed circa 1770) had revealed that the boy in the picture was really a girl. The x-ray analysis, done to verify the authenticity of the painting, had shown that beneath the top layer of paint Gainsborough had drawn a picture of a nude female standing in the same position. In its next issue, Uhu
claimed that this spoof had been "widely believed in art circles."
Uhu - May 1928