A bug is an erroneous behavior by a software program that was not intended by the program designer. In the case of World of Warcraft, this usually means things like a crash, lag, bad graphics, or an ability or spell that behaves unexpectedly or has a strange effect.
In 2009, Blizzard has traced a total of nearly 180 000 bugs

Some bugs have a temporary workaround, but will generally be fixed in upcoming patches.

Example:

• Around mid-February 2005, Blizzard announced that they had patched the server to prevent a bug in the paladin's Holy Light blessing that allowed it to be cast with a too low mana cost.

See Category:Bugs for more info.