This PDF is a recent rules list for official Pokemon TCG competition, so I'm pretty sure it's accurate.
According to section 11.1, a player that accidentally reveals the top card of their deck may be given a caution. Though not explicitly mentioned, the deck will also get shuffled.
According to section 11.2, a player that accidentally draws an extra card will be issued a warning.
According to section 11.5, a player that intentionally draws extra cards will be disqualified from their match and barred from the rest of the tournament.
If you're playing casually, the rules don't need to be that strict. See if you can work it out with your opponent instead -- the above rules are only "official" in the sense that they apply at sanctioned tournaments.