Glitches

Glitches (or bugs) are errors in a game or program that cause unexpected problems or unwanted behavior.

Causes
Glitches in The Sims series can be caused by any of the following:
 * Use of some cheats (notoriously Testingcheatsenabled).
 * Installation of official game updates/patches.
 * Use of game modifications.
 * Use of downloaded objects.
 * Hardware problems: A malfunctioning computer component or a computer that does not meet minimum game requirements.

Severity
Many glitches are so minor that they can be ignored, even if they occur often. Some glitches, while they do not corrupt any game data, may make the game partially or completely unplayable. Examples of these include graphical errors that cause the game to not render or display correctly. The most severe glitches may corrupt saved game data or cause a CTD (Crash To Desktop). Severe glitches also include Save game errors; bugs that can prevent a game from being saved.

Below is a list of how The Sims Wiki classifies glitches for use on this page:

Glitches
Please note that glitches are sorted by severity first and expansion pack/Patch second.