http://www.worldofmunchkin.com/faq/changelog.html

Starting with the 19th printing of Munchkin in May 2010, we made some changes to rules and cards across the Munchkin line. This page lists those changes for the sets that have already been released or are at print. We recommend bookmarking it and checking back frequently for new sets!

• The starting hand is now four Doors and four Treasures.
• We added a “Contradictions Between Cards and Rules” box, detailing the guideline that cards supersede rules. A few special rules always supersede card effects unless the card explicitly says otherwise.
• We explicitly state that you have a chance to play cards and alter your layout at the start of your turn before Kicking Open the Door. (That was always the intent, but the rules didn't make it clear.)
• Taking a monster's Treasure is no longer part of the Loot The Room phase. This removed ambiguity on several cards.
• The “friendly monster” clause was removed from the Loot The Room text. “Friendly” is never defined in the rules, and almost all monsters that could be considered friendly already say what to do if they do not fight, so this was both confusing and superfluous. The rule now is that if you encounter ANY monster, you cannot Loot The Room.
• Charity now explicitly says that you only give charity after playing (or otherwise getting rid of) all the cards you choose to. Additionally, we clarified that it depends on hand size only – the first version of the rules was not explicit about this.
• Combat now draws a clear distinction between Level and combat strength. We tried to remove the terms “base level” and “effective level” from the game, as they sometimes led to confusion.
• The rules for fighting multiple monsters now have their own subhead.
• “Asking for Help” clarifies the distinction between tossing cards into a fight and actually joining the combat, defining the term “helper” explicitly. It also says that the main fighter always draws the Treasure, even if it was the helper's ability that won the fight.
• Moved the “Interfering With Combat” section to a box under Combat.
• Running Away now ends with “Discard the monster(s).”
• Clarified the Death rules. You resurrect as soon as the next player's turn begins. At the start of YOUR next turn, draw four Doors and four Treasures and then take your turn normally.
• Added the rule that dead characters cannot receive cards or level up.
• Under Treasure, added the rule that Go Up a Level cards may be played on any player.
• New box: “Example of Combat, With Numbers and Everything.”
• New box about smartphone level counters.
• Added an explicit clarification that you cannot discard Items “just because.”
• Moved “Trading” under “Items.”
• Selling items now clarifies that you cannot sell less than 1,000 Gold Pieces worth.
• Changed “Treasures – Playing Them” and “Treasures – Using Them” to say “Items” rather than “Treasures.” Added an “Other Treasures” box.
• Added a “Super-Sized Munchkin” box and rewrote the Faster Play Rules.
• New! Steeds and Hirelings (except for Mooks) may no longer have Monster Enhancers played on them for a bonus.

• Super Munchkin now works like Half-Breed. It can be played with one class, giving all its advantages and none of its disadvantages. (Ultra Munchkin and Super Duper Munchkin have changed similarly.)
• Cheat! must be played with an Item card from your hand or on an Item card you already have in play. (This is not a change in intent, just a clarification in wording. Cheat With Both Hands has also been clarified.)
• Wandering Monster now says “add their combat strengths” rather than “add their levels.” Similar changes were made to MANY other cards that said “levels” when they meant combat strength.
• All Monster Enhancers have had “Level of” deleted from their modifier description. Negative Monster Enhancers no longer say “Minimum Level 1,” either.

• Wizard: The Charm Spell now requires a hand of at least three cards. You cannot discard a smaller hand to activate this power. To balance this, the Flight Spell now may be used after the Run Away roll.
• Cleric: Resurrection may be used to draw more than one face-up card, but all draws must happen at once (you cannot resurrect one card and then see whether you like the next one).
• Bard: A Bard may now use the Enthrall ability more than once, trying until he succeeds, gives up, or runs out of cards or opponents.
• Halfling: The Halfling no longer has a penalty to Run Away. A Halfling who fails his first Run Away attempt may discard one card to try once more.
• Gnome: The Gnome no longer has a penalty to Run Away. Gnomes gets a bonus with all items beginning with G or N. Monsters with “Nose” in the name never pursue Gnomes.

• Divine Intervention: Reworded to be more clear about exactly how the card works.
• Mate: Now duplicates Monster Enhancers only. Removed the Run Away penalty.
• Plutonium Dragon: Bad Stuff now says explicitly, “You are dead.”
• Doppleganger: Doubles combat strength.
• Hireling: +1 Bonus, can now use any one item for you, even if you couldn't otherwise use it. You can discard your Hireling (and the item he carries) for automatic escape from one monster.
• Kneepads of Allure: Restored to the 19th printing. Cannot be used on lower-Level players. Cannot be used to win the game.
• Magic Lamp: Previous printings had two copies of this card; one has been removed in the 19th and subsequent printings.
• Rat on a Stick: Now gives a +1 bonus and may be discarded from play for automatic escape even if the item is not in use.
• Really Impressive Title: Has NO value entry (not even “No Value”), and therefore is not considered an item.
• Tuba of Charm: Increased the Run Away bonus to +3, and you get to draw a Treasure if you Run Away successfully.

• Cheat! replaces one of the blank Door cards.
• Find a Map: Door cards, not Dungeon cards. (An actual erratum!)
• Contemplate Your Navel: Go up one level normally, or discard your entire hand (minimum 3 cards) to go up TWO levels.
• Siege Engine: You can ride in the Siege Engine at the start of combat. It does not count against Big items in play. If you do use the Siege Engine, you are -1 to Run Away.

• Curse! Exploding Kneecaps: Doesn't refer to specific items, just “any item specifically on the knees.”
• Freudian Slippers: Clarified that this is not a real sex change, and doesn't trigger effects that depend on sex changes.
• Senseless Act of Kindness: You can either go up a level or play it on a rival to force him to give you his highest-bonus Item and he goes up a level.
• Hireling: +1 Bonus, can now use any one item for you, even if you couldn't otherwise use it. You can discard your Hireling (and the item he carries) for automatic escape from one monster.

• New! The Duck of Earl results were changed so that higher results are better, so that Chicken On Your Head continues to be a curse.

• New! Politically Correct: Now affects items that have race or sex requirements and removes race/sex dependent bonuses from ALL monsters in combat.
• New! Grow Two Feet: Halflings and Gnomes no longer have Run Away penalties, so monsters with bonuses against Halflings or Gnomes lose those bonuses when this card is played. The rest of the card is unaffected.

• Rules: Revised to set off “Changing Dungeons Without a Portal” in a box.
• Dungeon of Comprehensive Curses: Any Curse that already affects the whole party is not affected by this Dungeon.

• Epic Junior: Add Accents and Loyalties to the types of cards that can be epicked.
• Theme Music: Should not have a value, not even “No Value.” Theme Music is not an Item.
• You'll Take Away My Toy When You Pry It From My Cold, Dead Hands: Add Accent and Loyalty to the protected classes.

* This set has been superseded by Munchkin 7 – Cheat With Both Hands.

• Epic Junior: Add Accents and Loyalties to the types of cards that can be epicked.
• Theme Music: Should not have a value, not even “No Value.” Theme Music is not an Item.
• You'll Take Away My Toy When You Pry It From My Cold, Dead Hands: Add Accent and Loyalty to the protected classes.

* This set has been superseded by Munchkin 7 – Cheat With Both Hands.

No changes!

• Evil Twin: Now duplicates Monster Enhancers only. Removed the Run Away penalty.

• Stunt Double: Now duplicates Monster Enhancers only. Removed the Run Away penalty.

• All references to “Credits” changed to “Gold Pieces.”
• …And Its Clone: Now duplicates Monster Enhancers only. Removed the Run Away penalty.

• All references to “Credits” changed to “Gold Pieces.”
• Autoflee 2000: Change -2 penalty to “No Bonus” and remove the text about costing levels in combat.

• Duplo-Ray: Now duplicates Monster Enhancers only. Removed the Run Away penalty.