The first thing you read in forums and helpdesks when you're experiencing a game issue is to remove/uninstall all the custom content you have, because 90% of the times files downloaded irresponsibly are the cause of the matter.

If it's true that no file is free from bugs and glitches as it is beta stuff, it is also true that a very few players read the installation and compatibility notes that modders publish together with their files. Incompatible or non-updated files have to coexist, thus, creating a huge amount of game issues that often cause the loss of lots, families and data.

In this regard, since the Sims 3 modding community is taking its first yet big steps, SimGuruJim from the development team has published a note on his official blog, which we'd like you to read and remember everytime you are experiencing troubles and complain about EA.

Hey Simmers,

We know there are a lot of new players out there, and we’re happy you’ve joined us in the Simming world! With all the new faces, we would like to take some time to talk to you about player created modifications to The Sims 3. While the creativity and dedication of The Sims fans never ceases to amaze us, we have noticed that some players are unwittingly using modifications (mods) without understanding some adverse reactions to their gameplay by this action.

There are oodles of fan-created mods out there. We simply cannot ensure compatibility of our evolving game with all of the mods being made available through 3rd parties. That means that if we release a game update and you have mods installed, the update might fail to install, your game could refuse to start, your mods could stop working, or your game could behave unpredictably after installing the update. If you aren’t too familiar with the modding scene for The Sims titles, here are some helping things to know:

Identifying Mods:

If you are asked to add, replace, or remove files from your installed game directory, (The Sims 3) you are probably installing a mod. If you overwrite installed files without making backups, you may end up having to reinstall your game down the line.
If you are being asked to install something that is not accessible through the game launcher or The Exchange, it could be a mod.
Make sure the source of the mod is trustworthy. You might be getting more than advertised. (virus or malware)

Responsible Modding:

We recommend that if you are going to use mods, please identify, and keep track of all of the mods that you install.
If you are asked to replace an original game file, make sure to back it up. You will want to restore that file to ensure compatibility with official game updates.

If you want to update your game with an official game update, it is highly recommended that you uninstall all mods beforehand.
It would be best to check with the mod creator to verify the necessity and compatibility of the mod post update. The mod may need to be rereleased to remain compatible with the evolving game or a mod may be unnecessary after an update (if the mod were designed to fix a bug that was also addressed in an official update).