Great General Glitch
I was playing a game yesterday and I got a great general on a tank army. On the next turn I moved the tank along the enemy road to the next enemy city and attacked it. I got a great general from that battle but I had realized that it already had one great general. I think the first great general lagged behind before the attack and the game thought it didn't have a great general. When I moved that tank army there were two great generals following it but it didn't give a 100% bonus for two great generals.
That's certainly a new twist on the Great General bug! Most of the time it just sits one square away from the unit, and can easily be wiped out by almost anything.
Don't expect to see it again though, because the underlying bug is supposed to be fixed with the 1.30 patch which is due out soon.