Glitches

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

 Tip: There's already ALOT of info on glitches, how to fix, how to report them and more on EA's Possible Bugs forum. You should head there first.

Causes
Glitches in The Sims series of games 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.

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.

Confirmed Sims 3 Glitches
If you want to add a glitch please go to the talk page and see if someone already reported it. If multiple people are experiencing the same bug it can be added to the list. If you can reference a website which reports this glitch, that would be best.
 * Unable to open Consignment shop buy menu - The game is supposed to return items to you which haven't been sold within 5 days, but it does not work. Instead the items start counting down into the minusses, preventing you from opening the buy menu. This can be fixed by cancelling your consignments.
 * Unable to open tomb door or loot adventure item - This can happen if you go too far into a tomb or teleport there without having the appropriate adventure.
 * Unable to excavate since Ambitions - People with Ambitions cannot excavate if the site is on a slope, the terrain has to be completely flat.
 * Grim Reaper quits his job - If you move the Grim Reaper in, there is a chance that he will not be replaced, and therefore will not come for a Sim if he/she dies. They will simply be lying there, unable to die but unable to live, in a state of limbo between life and death...

Old/Fixed Sims 3 Glitches

 * Save game error - (Error 12, Error 13, and Error 16), which prevents games from saving in The Sims 3. This was introduced in World Adventures. Patch 7 was released to fix this problem and Patch 8 introduced a recovery prompt for already corrupted games.