Crusader Kings and Europa Universalis have a diplomacy mechanic where you can "fabricate a claim" on a province, which allows you to wage a war of conquest for that province. The in-game explanation is that the diplomat researches (and bribes, and forges) a timeline by which the province should be rightfully yours (e.g. "proving" that 300 years ago your ancestor was the losing party in a disputed succession).
Are there any historical examples of such fabricated claims? (I suppose the hundred years war would be somewhere in the spectrum, but the claim here was still fairly legitimate and well documented, I'm looking for claims relying on more obscure/clearly falsified rationales)
Yes, of course. The forgery of diplomae to further property claims and the like was common, and recognized by contemporaries. This lead to to various devices, from rhythmatic prose (cursus) to chancery scripts to indenture.1 Many of these are outlined in Pope Innocent III's bull De crimine falsi (Regarding the Crime of Forgery).
The most famous medieval forgery is certainly the Donation of Constantine, a document purporting to be Constantine's gift of the Western Empire to the papacy, and thus one one of the cornerstones for all temporal papal power. In the fifteenth century, this was revealed to be a 9th c. forgery.
I would, however, advise some caution when thinking about the subject. Often as not (and I think in the case of the Donation), this wasn't some evil cleric twirling his mustache while scribbling out a forgery. Instead, you would have people inserting information about lands or rights they owned de facto but lacked the de jure proof. For example, if I have a document saying I own 4 farms, I might edit it slightly to include information about another one I own, just to make it safe. The result is still undeniably a forgery, but it qualitatively different than our current understanding of the term.
1 A verification device whereby two copies of a charter are drawn up on one piece of parchment, which is then divided with a jagged cut that looks like teeth (Lat: dens, dentis). The resulting documents have thus been "en-teethed" - indentured - and the term eventually came to mean a person bound by such a contract. This latter use survives long past the original diplomatic practice.