These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Crowning Moment of Funny: Not in game, but among the dev team: Planet 51 was created by Korin because it was his week to release an update...so he created that entire thing after locking the staff member who's ID is Warlic in a closet FOR FIVE DAYS.
as well as The "Mantis Shrine" quest in the Planet Yokai questline.
Demonic Spiders: Vladic knights on necryptos. It is almost literally impossible to get past them without a vampire hunter mech. Whereas previously most other vampire mooks were difficult, but not impossible, Knights can nerf your damage down into the single digits. Any stronger and they would rightly qualify as a boss.
Foe Yay: One Heroes' Heart Day event had a Shadowscythe Commander by the name of Medeia choose Major Braddock Steele as her mate. Un/fortunately, he rejects her stating that his true love is blowing up Shadowscythe.
For bonus points, one of your periodic missions as a cop involves taking down pizza delivery guys who step on people's cars.
Game Breaker: The monthly mech from June of 2009, the Warbear, sounds like your average mech...until you check its stats on the forums and discover the damage bonuses on the arm and shoulder weapons. Level 20 and below: 250%, Highest verison: 350%, everything in between: 300%. Not to mention the head weapon will boost attack and accuracy by an additional 10% each turn for 4 turns. The level 20 version could easily beat 2 level 28 enemies in a row. It can beat extreme mode bosses at more than half health. It can win the effing Star Captain bonus challenge at level 25. And the body weapon does guaranteed critical hits.
Then again, almost all monthly mecha classify as this.
There's a reason the MQ fans are the only fans in existence that complain about the player equipment being too strong.