I'm mostly asking because of Crusader Kings 2, to be honest, but I'm curious how many claims were fabricated back then. Who would create these? The nobility themselves, or their staff? What was their actual reason for war/what were they covering up? And how did we learn, ultimately, that the claims/reasons were fabricated?
It may not be quite the same thing, but here's a good example of sharp practice from the late middle ages that I came across recently. (From The Wars of the Roses by John Gillingham.)
At midnight on 29th March 1448, commissioners from France and England met to extend the terms of a truce for another two years. The documents were drawn up by the English and handed over "without so much as a candle" and the French didn't examine them too closely, assuming that they were the same as the previous terms.
Which they mostly were, except for one change. The list of allies and liegemen of the King of England now included the Duke of Brittany. In fact, he was a vassal of the King of France.
When English forces launched a raid into Brittany, they used this document to maintain it was a purely 'domestic' matter, between a vassal and his liege, and the King of France had no right to intervene. The King of France was not persuaded and declared himself no longer bound by the truce.
In this case the war went badly for the English, but had it been backed up by sufficient force, it's easy to imagine this document being the start of a process leading to England 'legally' annexing Brittany.
What makes this case even stranger is the fact that it seems like Henry VI, who generally wanted good relations with France, didn't know or approve of what was going on.