Here's a little compilation of bugs. Most of them have been there for a long time ; thus, I'll start with (hopefully) a couple of new ones.
New bugs
(Minor) Equipping a flag displays the weapon tactic as level 1 "exp_kind" without any way to gain experience in it. To clarify things, other handheld items use Grapple tactic just like the regular gauntlet or bare-handed attack, and allow for tactical experience gain.
(Minor) You must relog after doing the very last daily quest in order to update XP to 9999. Not just a visual bug, the game still believes you don't have full XP in your 99th level until you relog.
(Minor) Small Octopus stacks can be larger than 20 (same goes for other kinds of fish, thanks Kasumi_Chan)
(Minor) Shearing allows you to get full 20 wool stacks per click.
(Minor) Butcher NPC and Tailor NPC both sell stacks of 1000 wool, which also become stacks of 20 instead once in the player's inventory.
(Server) Attempting to make a rare item without the correct job item equipped wastes materials without any warning.
(Minor) Astrologer spell shop says your INT/WIS is not high enough when you actually lack WS/PS.
(Minor) Tailoring Quest NPC : At 55 Tailoring, the NPC asks for 30 Heart Breakers, then very politely repeats it wants 20. It actually wants 30.
↑ (Apologies, already reported by AcM : viewtopic.php?f=14&t=1615. Thanks, Tensu.)
Older bugs
Bounties
(Intended?) When someone completes a bounty, the bounty remains in the list and can be taken again, meaning the bounty board can be spammed if it isn't janitored regularly.
Taming
(Message) "You have succeeded in taming" appears when you have exactly 200 taming points instead of the progress, even when the monster actually requires a lot more points to be controlled. The monster does not get instantly tamed when this happens, though.
(Server) The taming point counter regularly gets reinitialized.
(Server) Tamed monsters sometimes refuse to obey commands ; the client sometimes says that "there is no tamed creature by that name" even though the target for /follow is right in front of you, alive, healthy and ready to rock.
Butchering
(Message) The smart observation "It seems cows leave meat and skin when they die" applies to every monster. Cows have taken over the world.
(Intended?) "There isn't anything more for you to gain" : Disregarding the grammar and logic errors, this is sometimes displayed even if you have not obtained any material from the butchered carcass yet, meaning that some otherwise valid carcasses simply cannot be butchered.
Farming
(Server) Planting seeds too close to each other or to the edges of a field may prevent you from harvesting some of them when they are ripe, without any warning message (just the usual failure SFX).
Cooking
(Minor) "Bottle with Milk", "Cup with Milk" and "Bottle with Water" items may be consumed, but none of them has any effect on Hunger. They only give the usual 1-2 HP back.
(Minor) "Cup with Water" and "Basket with Water" also appear to cause damage when swallowed. This is common sense, though

All secondary jobs
(Minor) Making a new bunch of items right after the item making countdown expires sometimes causes the action to be ignored and triggers the countdown again instead.
Apologies if some of these have already been reported.