Q: If I used the power of the Guardian of Forever to get an extra turn, then played Brain Transference, does my opponent now in my seat get the extra turn, or do I?

A: To review: By taking the Guardian of Forever (GoF) out of play in front of you, and putting it in the middle of the draw deck, you are granted an extra turn after your current one is finished. Brain Transference says you put your hand down, leave your Keepers on the table, and switch seats with one of your opponents: they get your hand, your Keepers, everything, and your turn ends immediately. The person who would normally go after your previous seat now takes their turn (or, in a two player game, the person now in your former seat).

This is a very tricky question which we debated for quite a while. It’s so complicated that we’re not even going to go into all of the questions that might hurtling around your head right now.

The official ruling from Andy is that the extra turn granted by the use of the GoF stays with the player who originally invoked it (we’ll call them Player A), not the opponent now in their seat. When does Player A get their extra turn? Right away after the Brain Transference abruptly ended their last turn, so, before anyone else goes.

It would go like this:
Player A has the GoF in play. As a free action, they take it out of play and insert it into the middle of the draw deck. They continue with their turn, and, for whatever reason, end up playing Brain Transference.

Player A picks Player B to “swap brains with” and both players set their hands down and switch seats, and Player A’s turn ends.

No matter where Player A is now sitting, they take the next turn: the extra one they gained by using the GoF.

Then the game continues as it would after the Brain Transference. If it’s a two-player game, then Player B, who is now in A’s former seat takes their turn. If there are more than two players, then whoever is sitting in the seat “after” Player A’s former location (we’ll call them Player C) takes their turn. (Playing clockwise, that would be the person to the left of Player A’s former seat.)

Q: What cards have effects that include ending my turn immediately if I play/use them?

A: Cards (Actions) that end your turn immediately if you play them:

Brain Transference: Star Fluxx
Time Portal: Star Fluxx
What Do You Want: Star Fluxx, Oz Fluxx, Doctor Who Fluxx
I’ll Be In My Bunk: Firefly Fluxx (This card does not specifically say that your turn ends immediately, but you certainly can’t continue your turn if you “Excuse yourself from the game and leave the room for a few minutes.”)

Cards (Rules) that end your turn immediately if you choose to utilize their ability:

Swap Plays for Draws
Get On With it

Since Rule-based Free Actions are optional, the player is not obligated to use their power, so simply putting them into play does not end the players turn immediately. However if any player chooses to use the powers of these cards, the effect is simultaneous with their turn ending immediately. Most notably, this means that if you draw any Creepers with your draws from Swap Plays or Get On With It, you’re stuck with them until your next turn, even if there are conditions which allow you to trash or give them away on your turn. Your turn ended immediately with the draw, so that window is over.

Q: What happens if you draw a turn-ending card when you use Wormhole?

…It says the card played does not count as a Draw or Play, so does it still end your turn?

A: Yes, while that card you draw from Wormhole (or any of its analogues) doesn’t count against the Draw or Play count as shown on the rule cards, it’s still part of your turn, and the card still counts as being fully played. Whatever it says happens, happens. That’s the risk you take, pulling a card out of the Wormhole! Keep in mind that the turn-ending effect of New Rules is optional, so simply playing them does not end your turn.

Also remember you can take the Wormhole option at ANY time during your turn: before your Draws and Plays, in the middle of your Draws, in the middle of your Plays, or after both, if you like. That’s the only control you get to exert: WHEN and WHETHER you decide to play a card from the Wormhole.

Analogues of Wormhole (which is in Star Fluxx) include (some with slight variations such as conditional requirements for use):
Magic Shoes in Oz Fluxx
Shiny! in Firefly Fluxx

There are many other Wormhole analogues in other versions, but those versions don’t also include turn-ending Actions.

See also: Q: What cards have effects that include ending my turn immediately if I play/use them?
See also: Q: What are all the different cards in different versions where you get to draw the top card and play it immediately?

Q: If I play Brain Transference and there is a Hand Limit, how does discarding work, if at all?

One player played the Brain Transference card, which states “your turn ends immediately.” At this time, the ‘Hand Limit 2’ New Rule was in effect. The player who played the Brain Transference card had more than 2 cards in his hand at the time the transference card was played.

Does this player discard down to the 2 card hand limit before switching places, or does the other player pick up the hand with more than 2 cards?

A: The two players would switch places, and the new player would get the large hand. Keep in mind, however, if it is their turn next, they don’t get to roll into their next turn with a giant hand. That giant hand is from a turn which is over, and whoever has it must discard down to the Hand Limit before starting their next turn.

Q: What happens if I play an Action that causes my turn to end immediately in the middle of Draw 3 Play 2 or Draw 2 & Use Em (or Fizzbin)?

…Since these cards are all played as part of a single play, would the player get to finish playing them out, or would their turn just stop? And if it just stopped, what would happen to any unplayed cards? We’ve been letting the player keep them.

A: The clear answer is that if you decide to play one of these turn-ending Actions first, of course your turn would end immediately, as you have been playing it. Of course, in the case of Fizzbin, you don’t get a choice as to the order of cards played, but that card most closely resembles D3P2/D2UE, since you set your main hand aside, and are working from a temporary hand of extra cards to execute the Fizzbin.

However, there is NO way that any remaining cards would go back into your set-aside hand. They are never intended to go into your actual hand at all, as indicated by the requirement to set your hand aside. Any cards left unplayed when you played the turn-ending card are discarded. If you wanted to play them, you should have done it before the turn-ending card.

Brain Transference: Discard remaining cards in your temporary hand and trade places with the player of your choice. Turn ends.
Time Portal: Choose a card as described and add to your set-aside hand. Discard remaining cards in your temporary hand. Turn ends.
What Do You Want: If you choose to take a Keeper or Goal out of the discard, it goes into your set aside hand. Discard remaining cards in your temporary hand. Turn ends.
I’ll Be In My Bunk: This card does not specifically say that your turn ends immediately, but you certainly can’t continue your turn if you “Excuse yourself from the game and leave the room for a few minutes.” Discard any cards remaining in your temporary hand. Leave the room.

Swap Plays For Draws and Get On With It, while they do involve having your turn end immediately, are New Rules, not Actions, so, as you can see from this answer, things would work a bit differently:
See: Can you Get On With It or Swap Plays For Draws with your cards from an Action like Draw 3 Play 2?

Q: If a card says “Your turn ends immediately,” but Play All is in effect, which takes precedence?

A: When you play an Action or use a New Rule card says “your turn ends immediately” it means it’s specifically overriding any Play rule that might otherwise require you to keep playing cards. You also end any option you may have to use Keeper powers or “free” Rule Actions. If it says “your turn ends immediately” then your turn ends immediately – so make sure you’re all done with stuff before you play/use one of these cards!

See: Q: What cards have effects that include ending my turn immediately if I play/use them?