Here is a gap in a famous series of papers.
G.H. Hardy, and J.E Littlewood Some problems in Partitio Numerorum, VII
Their series of papers "Partitio Numerorum" is quite influential in the development of the Hardy-Littlewood circle method.
Some comments on the missing part are on page 253 in a paper by R.C. Vaughan, Hardy's legacy to number theory, Journal of the Australian Mathematical Society (Series A) (1998), 65: 238-266. Cambridge University Press