As nasty as it may be to have, errata simply proves that everyone is only human, including game designers.  In all cases, errata only exists to make certain the game is balanced, even if your favorite card or character has just been watered down.  Unfortunately, sometimes not everyone is informed of a change in the rules.  Hopefully, with this information, I can help beginners to know about these changes.  All of the errata listed here is strictly official and is not my opinion in any way.
    If there is any errata that I have missed, please email me and I will add it if it is indeed official.
    I guess all of this could be found at overpower.com, but I wanted to have it here also.

    The green text is the official ruling and the white text is merely my personal remarks or history.
 

1.    Teamworks - You must make at least one follow up attack.  If, during the discard phase, you do not have the required Power card to make a follow up attack, then the Teamwork is considered an unusable card and must be placed or discarded.
    This was, I believe, the very first errata in OverPower, if you could call it that.  This new ruling is roughly mentioned in the latest rulebooks, so it is more widely known than the rest.
 

2.    Power Leech (BY) - When playing Power Leech, you must target one of the opponent's characters as an attack.  The power type is announced before the opponent plays a defensive action.  The opponent may defend Power Leech with a defensive Special card, if it would apply to any other attack with a Special card that is not a numerical attack.
    This card has been played several different ways over it's first year or so and I guess this turned out to be the most fair.  It can still be a very powerful card, but Battlesites usually have enough defense to prevent this from being the game winner that it used to be.
 

3.  Web Headed Wizard (BQ) - Web Headed Wizard may not be used to retrieve an Any Hero/Character Special.  It may also mot be used to retrieve an unusable card, like a Special for a character that has been KO'd.
    Most people were using 'Web Head' to get Power Leech a second time, which proved to be a little too abusive.
 

4.  Four Freedoms Plaza's Inherent Ability - Whenever you shift an attack using the Inherent Ability, you MUST defend the attack and you may ONLY play a defensive card that uses the power grid of the character.
    What this means is that you cannot use the Inherent to shift an attack to another character and then play a Special to avoid the attack.  You could use another card to shift the attack and then still use a Special card to defend, but not if it is shifted by the Inherent.  The only choices for defense would be Power cards, or any other cards that may be played with a Power card (Power card with a Basic Universe, Training, or Doubleshot).  Also, you must defend any attack that you shift, you cannot simply move the hit to someone else.
    This card has also been modified a few times and the latest seems to be the most fair.
    The +1 bonus to defense using Power cards for each power grid of 4 or less is not a cumulative effect and it was never intended to be.  This was not an errata to this card, it was more of a clarification.  Also, the bonus only applies to "pure" power cards of that type, never to Multi Power or Any Power cards.
 

5.  Fortress of Solitude and New Universe (EN) - In any battle when you Venture more than 2 cards, you may not play this Any Character.
    Basically, people quickly realized that if you drew this card, you could bet 7, or as many as you could, to make your opponent draw up to 5 cards only to plan on conceding and therefore wasting your opponent's cards.
 

6.  Trick Transport and Demonic Magick (DZ) - This card may not be used defensively.
    This card has gone through several different interpretations.  I remember specifically being told, a long time ago by someone official, one way it may not be played and then later I saw people playing it that same way because they were told it could be.  With the creation of the famous "Lock Out" decks the full potential of abuse was realized and this card was scaled back.
 

7.  Vertigo (LN) - This card may not be played defensively.
    This has a similar story.  Basically, in 1998 it was the backbone of about 75% of all decks.  Since this led to a decrease in deck diversity and generally made the game less fun, it was scaled back.
 

8.  The ill named Dead is Dead rule - For every character, regardless of Inherent Abilities or cards in play, if that character has meet the requirements for both a Cumulative and a Spectrum KO, then that character has been KO'd.
    What this means is, if you get 20 points and 3 power types on a character, then that character is KO'd.  For every character with an Inherent Ability or a Special card in play that modifies how that character can be KO'd, that only modifies that one condition of KO.  For example, Ghost Rider has an Inherent Ability that says he is Spectrum KO'd by four power types instead of three.  When it comes to Spectrum KO only, he needs four power types, but because of the Dead is Dead rule he still only needs three power types and 20 points.  Naturally, if Ghost Rider is hit with 20 points of damage anyway, he would be KO'd.  But if another card is played on Ghost Rider that modifies his Cumulative KO status, then it would only modify that one condition by itself.
 

9.  Guess my duration? - For Special cards, if the duration is not specified on the card and it is a One Per Deck, then it's duration is for remainder of game.  If it is not a One Per Deck card and the duration is not specified, then it is for remainder of battle only.  This also applies to Artifact cards that do not specify a duration.  Maverick's Freelance Spy should have been printed to include the text "for remainder of battle." and is not affected by this errata.
    There was some confusion over the cards for Mole Man and Leader that forced all attacks by the opponent to be made against a certain character.  This was made even more confusing by the One Per Deck version for Grunge.  This errata clears this up.  It also explains why Onslaught's Special Cannon Fodder is for remainder of game [or until chosen teammate is KO'd, whichever comes first].
    This errata also applies to Artifact cards, namely Image Inducer which was being abused.  The Siege Perilous states "each battle" which implies a duration of remainder of game.  The other non-OPD Artifact cards, Linkstone, Adamantium, and Ultimate Nullifier, are used rarely and are not grossly hindered by this errata.
 

10. Any Power 7 from Classic - This card should have had "One Per Deck" printed on it and is to be considered a One Per Deck.