A "tin shed" is an Australian term used by the stamping community to describe printing flaws in many of our pre 1930's stamp that happened because of the terrible working, printing conditions, the printers of the day had to work under. The 1st stamp printing work shop for Australia's 1st stamps was in a very old tin shed in Melbourne, with holes that the wind could blow in. The tin shed would get very, very hot in summer and very, very cold in winter. Some of the constant printing flaws were made by rats that had a chew on the printing plates. I have seen pictures of the printers working on the foot path outside of the shed and the mind comes up with many reasons why they would do such a thing.
In summer the gum from the top sheet would leave a gum stain of the top of the bottom sheet and then was printed on so when the stamp was soaked the gummed area would lift the print of the stamp with it. As well as dust, paper lint etc. Thus causing a "Tin Shed".