A particularly famous example of a fabricated document is the so-called Donation of Constantine which was likely drafted in the 8th century in order to assist the Carolingians in consolidating their power and to explicitly claim that Constantine I gave complete control over the issuance of the imperial title to the the Pope. In terms of land the document claimed that Constantine I gifted numerous estates across the Empire directly to the Pope, which may have been used as the basis to justify Pepin the Short's donation of land seized from the Lombards to the Bishopric of Rome, resulting in the creation the Papal States. As far as being used to justify the seizure of land, Urban II used it in 1091 to make a claim for control of Corsica.

The language of the document had some even more important implications in terms of defining (or confusing) the relationship between the Pope, the Eastern Roman Emperor, and the five episcopal sees. The document claimed that the See of Rome possessed a higher spiritual authority than the other four Episcopal Sees and challenged the authority of the Eastern Roman Emperor in theological matters, which contributed heavily to the debates which led up to the Great Schism in 1054.

During the 17th century, Louis XIV introduced the "policy of Reunions", where he could constantly expand French borders by annexations of small parts of the HRE on the basis of flimsy claims, some of them going back well into the Middle Ages.

Teutonic Order fabricated a donation of HRE emperor in order to fight the Polish claims to their territories.

During the partitions of Poland, Austrians justified their annexations by reviving the ancient title of Galich-Vladimir (in Latinized form Galicia & Londomeria), which they claimed was their rightful inheritance.

From my reading, and I stress that I'm not an expert, the impression that I get is that forgeries were pretty common, albeit typically not the broad territorial transfers that CKII works with. On the most basic level, almost every work I've read that mentions medieval agreements or charters highlights the possibility or reality of later alterations or outright forgeries. Charters were particularly subject to tampering, given the high legal importance attached to them when it came to determining land ownership. The clergy were prime offenders here, both due to their privileged position as a literate caste and the growth of monasteries as economic/landowning centres in their own right.

While I believe that these local cases of land ownership were probably the most common, there were many other more high-profile cases. Documents purporting to show a local connection between a saint and a locality - as in the famous case of Ademar and St Martial [1] - was a lucrative business. And of course the Papacy often conveniently 'discovered' new documents in its archives that happened to strengthen its position in disputes; the Donation is the most obvious but also see the impressive collection of fake letters and laws called the Pseudo-Isidorian Decretals. In the secular realm, a notable forgery is the 14th C Privilegium Maius which was used to elevate Hapsburg Austria to an archduchy.

[1] In short, Ademar, a local monk and historian in 11th C Limoges, develops a set of forgeries that show that the local saint, St Martial, was actually one of the original Apostles. It's a crazy tale that requires moving Martial's life by a few centuries and inventing a whole load of new miracles for him. But the local bishop and church go along with it and schedule a big mass to celebrate this discovery.

However an Italian monk just happens to be passing by and he not just spots the idiocy of the notion but calls Ademar out during the ceremony, in front of the whole church. Ademar's reputation is ruined and while he spends years 'compiling' yet more false documents to try and 'prove' his case, ultimately he leaves for the Holy Land in disgrace. Ironically, the forgeries he leaves behind are so good that they convince later generations and for centuries St Martial was celebrated in Limoges as an Apostle.

As a follow up:

Another tactic encouraged by CK2 is searching for disgruntled minor claimants to territories you want, them pressing their claims in ways that leave you in control of the territory.

It's such a big part of the game that I have to imagine it is based at least loosely on historical examples, but I've never heard of any such examples. When and how did this sort of thing happen?