I had one just a few hours ago. I was playing Rome: Total War (because I'm pretty much out of games until the holiday season) and I was defending one of my cities from barbarians. I had a nice city, with stone walls, and a good, well rounded army that they didn't have a chance getting past. After rearranging my troops to stop them cold at the walls, I start the battle and the enemy troops just start running. At first I thought that they were just withdrawing, because they knew that they had no chance, but I was still suspicious. I double-checked the gates to make sure that a spy hadn't opened them, they were closed. I took another look at the walls on the side that they were heading towards and it turned out that there was a huge, gaping hole on the other side of the city, far away from any of my soldiers. Out of frustration, I quit the battle and reloaded the last auto-save.
Turned out that, if you build new walls over damaged ones, the new walls will have permenant holes that cannot be repaired. I'm gonna have to look to see if the glitch is fixed in a patch and hope that it resolves it.