In feudal Europe "claims fabrication" really happened many times - for example, Teutonic Order fabricated a document, supposedly signed by Mazovian duke in early XII century, which granted the Order the rights to all lands conquered from pagans in Prussia. In reality, that duke (somewhat naively) believed that noble and pious crusader monks will give him all the land they will "free" from pagan rule. After the duke died, Teutonic Order used that forged document to justify constant expansion in the Baltics, and thus Polish-Teutonic feud started. Later, even more bold move by the Order happened in 1306 - Polish duke was busy in other part of the country, when suddenly Brandenburgians invaded and besieged Gdańsk (Danzig), and that duke paid many to The Teutonic Order to chase Brandenburgians away (basically, in gameplay terms, he rented condottieri). Teutons did chase Brandenburgians away, and afterwards laid siege to Gdańsk themselves, took the city and slaughtered the inhabitants. So, in some instances they didin't bother with claims...
On average, medieval Europe was very concerned with justified ownership of land - but, there was a legal concept of "right of conquest" - meaning that if you conquered land by force, it is yours, and it's legal ! Basically, in medieval period, inheritance and conquest were the most important ways to acquire new lands. That's why burgundian inheritance is such a vital event - there was no true separation between a private property of a ruler and a state as a legal concept, so you could inherit a country, which is unthinkable today. It was changing towards the end of middle ages, and in XIV century a concept of "Crown" started to be very important - "Crown" meaning all lands, laws etc. associated with a title of a king. Thus, you can hear of "Lands of Bohemian Crown", or Aragonian Crown etc. Thus, if it was commonly beleived that some land was rightfully part of that crown, and in reality was in somebody elses hands, it was a good casus belli.
Basically, it was the matter of international law - with time, it became more and more refined, and stabilty was greater and more valued - in CK2 period there is a concept of adventurers, and it really happeden that some guy just took his private army, invaded somthing and started a new state (like Norman Sicily). In EU4 period such things were not happening anymore.
Other continents is another thing altogether - Europe was really divided into small "provinces" like in EU4 or Ck2, with rights and privileges attached to them, and complicated system of feudal vassalage. In India and China, things were not based on clearly defined territorial entities, to which rulers had some legal rights - it was more prestige based in Asia, and it didn't really matter what land or culture some ruler is governing, his dynasty's prestige was most important - that's why you have countries like Bahmanis or Timurids there, and in Europe you play France and not Valois, or Poland and not Jagiellons (an exception would be Habsburgs).