I recently talked to someone from South Tyrol in northern Italy.
They insisted that Italy had gained the territory when they signed a peace deal with Austria after WWI. Due to a misunderstanding about when the deal took effect, the Austrian army began to march home before it was legally enacted. The Italians then occupied South Tyrol with no opposition.
The Wikipedia entry for South Tyrol makes no mention of this. Any truth to it?
This answer is on the shorter side because there's not too much to say, but the long and short of it is no, there is no truth to it. South Tyrol was slated for Italian annexation in 1915 through Italy's signing of the Treaty of London with the Entente as terms for Italy's entry into the war on the side of the Entente. Article 4 of the treaty states:
Under the Treaty of Peace Italy shall obtain the Trentino, Cisalpine Tyrol with its geographical and natural frontier (the Brenner frontier), as well as Trieste, the counties of Gorizia and Gradisca, all Istria as far as the Quarnero and including Volosca and the Istrian islands of Cherso and Lussin, as well as the small islands of Plavnik, Unie, Canidole, Palazzuoli, San Pietro di Nembi, Asinello, Gruica, and the neighbouring islets.
During the signing of the Treaty of Saint-Germain-en-Laye in 1919 between the Entente and Austria, South Tyrol was formally annexed by Italy in line with the Treaty of London's allotted territories. Italy didn't get all the territories promised to it due to conflicts with the new Kingdom of Yugoslavia in the Adriatic, but South Tyrol was formally ceded to Italy as agreed with the Entente.