The right answer, regardless of REL, starts with "call a judge."
Consequences to rules violations questions always vary depending on the REL (Rules Enforcement Level) that the event is operating under. Prereleases are held at Regular REL, so the IPG (Infraction Procedure Guide) does not apply there - only at Competitive REL.
There is a guide to judging at Regular REL, which refers to the following for miscellaneous mistakes:
A player makes an in-game error not mentioned above
This will cover the bulk of player errors, and we will usually leave the game as it is. Fix anything that is currently illegal (e.g. an Aura enchanting an illegal permanent or a card in the wrong zone) and continue the game. If the error was caught soon after and rewinding is relatively easy, you may choose to undo all the actions back to the point that the illegal action happened. This can include returning random cards from the hand to the library to undo card draws (though don't shuffle the library if you do this!), and reversing various other actions (such as declaring attackers or blockers). Don't go crazy with this!
At the judge's option, he or she can alter the existing game state to best adjust for the mistake. That same guide also calls out:
Keep in mind that the philosophy or fixes found in other documents (such as the Infraction Procedure Guide) are designed for a stricter environment and should not be applied at a tournament run at Regular REL.
There are a number of other articles published by Wizards about judging at Regular REL - see both http://www.wizards.com/Magic/Magazine/Article.aspx?x=judge/article/20080605a and http://www.wizards.com/magic/magazine/article.aspx?x=judge/article/20101111a.