A bug is an error, flaw, mistake, failure, problem or fault in a the game that prevents it from working as intended. This may include things such as graphical flaws, spells not working correctly or features not working properly.
Bugs are often fixed in game updates. If you find a bug you can use the /bug console command to report it, or visit the support section of the official Guild Wars homepage for more information. The official homepage also has specific information on how to report a bug.
for a current list of known bugs, see Category:Bugs