Below is a listing of various easter eggs within the game.
When you get a ship drop, extend the flash object and click on the upper left third of the black area (the cursor will change to a hand) to get the COMPASS-CHAN CLASSICAL DANCE PARTY.
It seems to have been removed on December 2014. Instead it was replaced with an image of a black colored ship girl with NO DATA printed on the anthropomorphise version. Since sometime around then till now, the NO DATA variant has now been removed for just a simple background, the voice line, voices and the caption but no actual image of the ship girl herself.
Some more information about it can be found here (albeit in Japanese only): http://kancolle.doorblog.jp/archives/35481633.html
As well some Google search results (albeit again in Japanese): http://goo.gl/L8BYDG Extended version of the "compass-chan classical dance party" music is found here: https://youtu.be/5TOzc4MfM9I
Non-functional repair cancellation dialog
The "cancel repair" dialog is a function that is likely removed from the game. After placing ships in the repair dock, stretch the game window horizontally until there's a significant amount of wide black space on the left and right sides. Clicking on the space on the left will display the dialog, which does not actually cancel anything if "yes" is selected. Note that selecting "yes" also causes various bugs to occur.
Re-clicking the black space again (by cancelling the window for the first time and clicking on the black space again) will sometimes show not an "undefined" ship but also a random ship that is currently in your possession (it can even show a ship that is currently going on an expedition).
Bug bouncing in the visual gallery
The fairy (with the books on her head) in the gallery also has a bug on top. If you click on the fairy, the bug will bounce, and sometimes fly away. There has been a mini-game where some people see how many times they can click on the bug before it flies off (record on the Japanese Wiki seems to be 78 times).
Jumping Construction Girl 
This common easter egg only happens when you begin constructing a Ship girl, and clicking on that particular construction girl with a ponytail carrying boxes.
By taking apart the flash game client, one can discover features that are unimplemented, or have been cancelled.
- 補給艦 (Supply ship)
- 超弩級戦艦 (Super dreadnought)
- Similar to BB.
- Historically, almost all battleships (except Gangut) in the game are super dreadnought.
- VT信管/近接信管 (VT fuse/Proximity fuse)
- Usable by all ships except submarines.
- Appeared on June 12, 2013 update notice together with other AA equipments when it mention equipment AA power increase, with a note that it is unimplemented.
- Available from extraction for many ship implemented in the game's first 1.5 years but can never be heard in-game.
|Introduction||初めまして、どうぞ宜しくお願いいたします。||Hello, please to meet you.|
|Secretary 1||何かご用でしょうか。||Need something from me?|
|Secretary 2||はい、頑張ります！||Yes, I'll do my best!|
|Secretary 3||提督、作戦準備の方は、大丈夫ですか？||Admiral, are you ok with the preparations towards strategy?|
|Secretary Married||やったー！||Did itー！|
|Looking At Scores||提督に通信が入っています。|
|Joining A Fleet||艦隊、出撃します。||Sally forth, fleet.|
|Equipment 1||強化ですか？ありがとうございます！||Strengthening? Thank you!|
|Equipment 2||どんどん強くなりますね！||I'm becoming stronger!|
|Equipment 3||やったー！||Did itー！|
|⇧ shared with expedition selection, resource collection, instant repair and development|
|Supply||強化ですか？ありがとうございます！||Strengthening? Thank you!|
|Docking Minor||修理に入ります。||I'm going in for repairs.|
|Docking Major||ドック入りします。||I'm going into dock.|
|Docking Complete||修理が終わった艦がありますね。||Ship has been repaired.|
|Returning From Sortie||艦隊が母港に帰投しました。||Fleet has retained back to base.|
|Starting A Sortie||両舷全速、出撃です！|
|Attack||敵艦発見！てぇぇェェッ！||Enemy ship spotted! Fiiree!|
|⇧ shared with day/night special attacks, support expedition team arrival|
|Minor Damage 1||あぁはぁいッ！|
|Minor Damage 2||いったったったったったったあ…|
|Sunk||こ、こんな所で…||At a place like t-this...|
|Equipment category selection screen at development screen|
|In-game message sprites
|Sortie node events
- Gold Coin (presumably to show how much DMM point that player have), more detailed stats for kanmusu, abyssal different levels (just like PvP):
- See early screenshots released on numerous sites like those reposted by http://kanmusu.blomaga.jp/articles/53681.html and http://www.4gamer.net/games/205/G020591/20130228067/