Found a really strange, kind of game-breaking bug today. From what I can tell, it was caused by trade deals not expiring properly in terms of trade for strategic resources.
I didn't notice it at first, but what seemed to happen was, I traded 5 horses to Arabia. At some point, I got the message that my deal had ended and I got my 5 horses back. Then, the next turn, I got a message that the deal had ended, and I got another five horses... then another... then another. I'm not sure how long this went on, but I didn't realize something was really wrong until I noticed was getting a surplus of over 100 horses! Before then, I was playing an expansionistic game, so I just chalked the high numbers up to the war I was having with Germany where I wasn't using any horses (he had lots of spears).
Eventually, Arabia was willing to trade away every luxury they had, including some single luxuries, and give me massive piles of gold in exchange for dozens of horses, which would then result in dozens of more coming back to me. To save the quality of my game, I stopped trading horses entirely, but the bug continued until I had well over 2000 horses.
I've attached a save file and a screen shot from when they were around 500.
It's likely that the other resources were doing the same thing (I seem to have a lot more coal than I should). I was playing a game with mods, but none of them should have touched trade numbers or resource totals. I only had a few balance tweaks, and a ui enhancement. The save file is base-game compatible.
View attachment Too Many Horses!.Civ5Save
Edit: Whoops! Didn't know this one was so popular! Sorry!