That's why they all tell you to use their products at your own risk.
If only EA games could (would?) make the same statement

Recently, I was hit by this infamous Invisible Sims Bug. I find myself lucky enough that it's just very recent, although I have some idea about why I had never met this behavior before (but it's nothing I'm allowed to speak of here). I did some research to understand what was happening. From all the tidbits of info I found around the web, it seams that this bug has indeed always existed. It was just much more rare in the base game than now. Every EP has increased the occurrence/probability of being hit by it. In fact, it's probably every patch too.
Why? The bug is in the code. A game is just a software, pieces of code. So that must be where the problem lies, since we are all speaking of unmodded games. This is just my guess, I'm not a programmer and I don't work at EA games, but there are probably several 'small' bugs lying around. Each EP adds more code... and more bugs! At some point, there is some kind of resonance or snow ball effect, all those bugs generate bigger ones => eventually invisible Sims and the likes.
If the bug is in the code, its effects are registered in the save files, these become 'corrupted'. The game (software) can't handle correctly what's written in those files. Once the files reach that point, they're just lost. There is no way of fixing them.
What could explain the fact that not every players experience it the same way, or as often as others, is that we all play in very different ways. The longest you play a save, the more bugs are registered. The more your Sims interact with their environment, the higher probability for the game to do something wrong (like in the Rabbit Hole challenge for instance).
Then, the whole patching process doesn't seem very stable. Even EA games acknowledges it and advices to unistall the game completely - with registry clean up - once in a while, and preferably with every patch.
Well, that's a long rant! If only they could put more developers on the task...