I’ve had that issue countless times where it caused weird bugs and annoying crashes to my game. So you must be careful and read about what you’re downloading. But that also means CC from previous versions might harm your game as well.
Most important thing to do before downloading any CC, is to make sure your game is fully patched to the latest version to prevent any unwanted behavior caused by CC from later versions. This is a basic summary of the steps to take to download and install custom content for The Sims 3.
As requested, a simple guide explaining how to install Custom Content in The Sims 3… Summary
