Quote Originally Posted by Jxrc
Happened to me a few times but nothing huge (usually it was due to the absence of an heir and the "royal general" fighting for the throne).

Worst thing that happened to me was in a Sicilian campaign. Was about 1260 and I had all of Italy, Spain, North Africa and large part of France. Decided to build crusades in Normandy and created several markers. Chapter house was destroyed by and earthquake (lucky me ...) and one marker was destroyed. Enough prestige to avoid rebellion. Next turn, second marker disappear (Why on earth do they no all disappear in one turn ? Perhaps because you can only have one crusade going, and thus failing, at the time ?). Result: civil war. Side with the rebels who retained Normandy so that the following turn another marker was lost !!! Hurray !!! Second civil war in a row. Ended up with a one star king proud owner of Malta, Sicily (with tow units of peasants and a bloody huge stack of rebels in Naples), Provence, Corsica and Genoa ... I have stopped the game after a few more turns ... which was so silly of me since I lost the pre-chapter house destruction auto-save...
....And that's precisely why I usually only build Crusade/Jihad markers as I need them. Too risky otherwise.

Civil wars can be a great way to shake up a game, particularly if your faction has reached the point of "critical mass" (where you're so big and powerful that you know it's only a matter of time before you win the game). They can also be an effective way to get rid of a weak faction leader, or even the whole royal family (if they all stink).

Of course, civil wars also tend to be more fun if you deliberately instigate them, as opposed to having them occur due to unfortunate (and/or unplanned) circumstances. I've purposely started civil wars as the Danes, the Norwegians (in XL), the French, the Eggies, and the Byzantines, usually with pretty good results. Sure, I lose half my territory at first and had to fight to gain it all back -- which isn't always possible, as other factions often will nab some of your provinces that have now become "rebel" -- but I still find it's usually worth it in the end. There's nothing like a few years of constant campaigning against your erstwhile brothers to bring vigor to your (new or existing) royal family.