Jarrema, it is just happenstance that it hasn't happened to you. Could be the Civs you play or some other pet setting/style makes it more unlikely that you'll see one, but given enough tries that specific trigger(s) could happen and you'll be left wondering if you need a new computer or you've hit a Waiting on Civ (WoC).
Personally, I've had none in the current 8.34 but I wouldn't accept that all the WoC bugs have been squashed. Certainly Sephi's recommendation is the best first response, that is, make sure you have the latest revision because many WoC have been reported and fixed with his latest rev.
To find your suspected WoC, first reload/restart CIV to make sure it is repeatable. To fix you should do what AngelGabriel said to do, manually start elliminating down to the cause. You do it step by step, checking between each step to see if the problem has gone away. To start, each step shoule be big, like remove an entire Civilization(s). If removing a Civ works then bore down into that Civ. You could start by seeing what they've done in that turn that may have triggered the WoC. Or, you could look for and remove Hidden Nationality units in that Civ (usual suspects). Continue this methodical whittling down to a prime suspect who's removal allows you to continue the game with the least amount of impact.
Of course, if the event/unit is one that Civ is interested in they may trigger another WoC when it is repeated again, and again, again but who knows... maybe not.
Check out all of Breunor's posts in the Civ4 - Fall from Heaven thread. He's a master at this sleuthing.
Please, always report these events along with a game save in the Bug thread. If you can post your investigation results as well, all the better.
Good luck
