A-to-Z-Dragon Buster Cannon
MST
Mystical Space Typhoon
I hope you don't need your GY... or that card you just searched.
Masked HERO Dark Law
Judge! Why is my opponent is saying that I can't respond to their trap card with my normal spell even though it's my turn??
Spell Speed
The trap card Counter Counter has the simple effect that it negates the activation of a counter trap and destroys it. However, it was completely unplayable until an errata changed it to one of these.
Counter Trap
A card that flips an opponent's monster face down
Book
ROTA
Reinforcement of the Army
Strike me down and I'll become more powerful than you can possibly imagine... Oh D.D. Crow? Yeah okay that'll kill me.
Destiny HERO - Destroyer Phoenix Enforcer
Judge! My opponent negated my Pot of Prosperity, why are they saying my monsters are still banished??
Cost
Cards whose effects make them "always treated" as another card extend far beyond the game itself, and actually affect deck building as well. The most notable example being this field spell, and its five copycats.
Umi
A monster that searches within its own archetype on summon
Stratos
DAILY DOUBLE:
UDF
Number F0: Utopic Draco Future
Wait I can pop back row too? How was I ever released in 2007??
Elemental HERO Stratos
Judge! I think my opponent cheated, they summoned one of these monsters by using one of mine as a tribute, and they said I couldn't even respond to it!
Kaiju
A slimy level 2 aqua monster had its name changed in 2016 to avoid confusion with another archetype, however many of the old cards from that time still deadname the card, excluding it from effects, stating "(except ___________)".
Frog the Jam
A card that send another card from the deck to the graveyard
Foolish
DAD
Dark Armed Dragon
Siiiighhhhhhhh, no I don't gain extra attacks for each fusion material used, only for each fusion monster used as material... I'm not happy about it either.
Wake Up Your Elemental HERO
Judge! My opponent's extra deck sleeves are different colours, I want them to receive a warning, they could be running this draw spell!
Pot of Extravagance
Elemental Hero Burstinatrix is no stranger to changes, having her artwork changed when coming over from the OCG to have her clothing less revealing. However, she is also the only normal monster to ever have her flavour text errata'd. Having it changed from saying she was "the only woman among the elemental heroes" to being "the first woman among the elemental heroes" after this HERO was released.
Elemental HERO Shadow Mist
A card you never want to see in your hand
Garnet
PS1, PS2, PS3, PS4, PS5
Pendulum summon
I'm like Naturia Beast but better! Wait, what are you doing with that Linkuriboh...
Destiny HERO - Dark Angel
Judge! What do you mean I can't Solemn Judgement Destiny Hero Malicious's effect? They're using it to special summon a monster, and Judgement negates summons!
Inherent summon (A summon that does not start a chain)
Before its errata, Hundred Eyes Dragon originally contained a hyphen in its name, which inadvertently allowed it to be searchable in this fan favourite archetype.
Red-Eyes
A monster who is unaffected by card effects
Towers