Playing an extra supporter during a turn...

[aade8]

I had this situation come up today: Player A played a Roseanne's Research near the beginning of his turn. After going through some gameplay motions in the same turn, he layed down a Buck's Training and proceeded to draw two cards.

Since he saw the cards he drew, I concluded that the action could not be satisfactorily reversed. Does this count as a major, or severe gameplay error?

 

[PokePop]

"reversing" or "correcting" a game state does not require you to turn back time and negate what happened.
If that were the case, every single thing would be irreversible.

All that has to be done to correct or reverse a game state is to bring it back to a state that if you took a snapshot with a camera, it would look the same as it did before, or look as it should look.
Knowing two cards on top is a minor thing. It can be corrected by showing those two cards to the opponent, for example.

Take a look at the examples of Game Play Error Severe that are found in the Penatly Guidelines:

Examples of Game‐Play Error: Severe include:
• Shuffling your hand into your deck without the use of a card effect.
• Shuffling your Prize cards into your deck without the use of a card effect.
• Shuffling your discard pile into your deck without the use of a card effect.
• Picking up your cards before both players agree on the game winner.

What you see here are actions that have done severe damage to the gamestate. Entire areas of the play area have been wiped out in these examples. Does that sound like two extra cards being drawn? No, not at all.

Now, you don't state if it can be reliably determined what the two cards were. If they can be determined, for example, he hadn't put them in his hand along with his other cards, or the other cards in his hand are known due to other reasons, then the play is 100% rewindable and the gamestate can be 100% restored. That would be a Minor error at that point.

On the other hand, if it difficult to determine which cards were drawn, it might be proper to count it as a Major error.
This will take more judgement from the judge to determine and also to decide on a "fix".
Should two cards be chosen at random to be put back in?
Should you go with the two cards indicated by player?
Should an additional penalty be applied, such as prize loss or multi prize loss, to compensate for the potential damage to the gamestate?
This is where a good judge earns his packs. And I can't give you a one-size-fits-all answer to that question here.