From WoWWiki

Bug

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.

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.