After the second consecutive game where I unexpectedly side-switched one of my opponent's characters, he got kind of frustrated, so I decided to make a list of the cards that have side-switching or other similar game-changing surprise powers. Obviously, the definition of a "gotcha" is somewhat subjective; there are other cards that could be on this list.
Some people might not want to read this list, if you prefer to be surprised by cards you didn't know about.
Crossed Swords, and a Kiss (SECRET)
If at the end of Melee, the owner of this secret is a Man or Woman who is alone in the space with an enemy character of the opposite sex (Man or Woman), and both characters are at less than Max Health, and both attempted a melee attack on the other in this Melee, the enemy character switches sides.
An Enemy Turned (SECRET)
If an enemy with 3 or less Honor enters the HQ or Prison site of your team base, he immediately switches sides. This can also be triggered if such an enemy is imprisoned. If so, he switches sides and is no longer imprisoned.
What Have I Done? (SECRET)
If an enemy with 5+ Honor kills (not imprisons) an allied character with 5+ Honor, that enemy switches sides, suffers 1 damage, loses all cards, and is banished.
Charm Cube (SECRET)
Play just before an enemy attempts a ranged or melee Hit roll against owner. Mental ability. Challenge Wits vs attacker. On Amaze, attacker switches sides.
Treachery & Loyalty (SECRET)
Play at Lith's Chamber in the Adventure Phase. Challenge Honor. On Success/Amaze, regain control of any switched characters.
Emergency Body Doc (ELITE)
Free Action: Revives one non-Machine character that died in user's space, restoring 1 Health.
Booby Trap (SECRET)
If picked up off the ground or stolen from the enemy Vault, the Booby Trap explodes, causing 10 damage to the one who disturbed it.
Use at start of a Melee. Enemies in the user's space drop their cards in the space and are moved into one adjacent space, user's choice. Sentinels are instead destroyed.
Can use immediately when an enemy triggers a Cube. The enemy cube fails and the failed user suffers 2 Damage. That cube and all revealed enemy cubes are destroyed.
Fluffy the Cat, Longhorn Prime, Smoot, Synetic Override (SECRET)
One-Legged Rooster (COMMON)
Branding Iron (ELITE)
These cards allow their owner to take control of enemy Pets or Bots. Synetic Override affects Bots, the others affect Pets. Branding Iron is permanent, not single-use, and can affect unrevealed Pets (and Mounts).
I turned Old & Chaotic Evil Bob to Lawful Good (for a day...)
Thank you for sharing. That list could be very helpful, and while I didn't read beyond the explanation, I AM bookmarking it!