The responsibility to make sure that everything is legal in an individual game falls down to the players themselves. While we do not expect players to know every aspect of the rules, we do want them to keep track of the game and make efforts that nothing illegal happens. This applies to making efforts so you don’t make an illegal play and to paying attention to the game in general so your opponent does not play incorrectly. As always, if there is ever a question, please call a judge.
In instances where something illegal has happened, a judge should also be called. The Magic Infraction Procedure Guide covers a lot of different types of errors, and how to correct them. Most penalties are given to the player that made the error. However one, Failure to Maintain Game state is given to other players in the game that should have noticed the error. This penalty is a warning at all levels, and it should not be upgraded. The reason for this is we do not want players to hold back when they notice an error made by an opponent. We want players to be vigilant and to pay attention, but do not want to punish a player for not realizing an opponent has goofed up.
Keep in mind that if a player were to notice an error, but not call a judge because allowing the error was beneficial to himself this falls under ‘Cheating- Fraud.’ Technically speaking, this penalty is also not upgraded, but that’s because the standard penalty is a disqualification.