With the last point in mind, illegal acts for which the Player can claim forfeit such as incorrect moving of the Pieces, allowing mobile devices to disturb the game, obvious cheating etc.
In such occurrences, it is probably best to raise your hand and ask for an Adjudicator to resolve any disputes.

Another key rule to be aware of is "Touch Piece, Move Piece" where if a Player touches a Piece on their turn, they must move it.
If an Opponent's Piece is touched, the Player must make all endeavours to capture it. 
If multiple Pieces are touched, the Opponent chooses which Piece is Moved or Captured.
In Serious Games, Rules become brutally unforgiving so it makes best practice to get into the habit of thinking before picking up Pieces and only when a move is fully decided upon.

Another unwritten reason which can lead to Won Games is where a Player makes a blunder which can vary in severity. For example, silly mistakes such leaving Pieces vulnerable to capture, miscalculating Exchanges that lose material in trade overall, moving Key pieces only to allow the Opponent execute a devastating tactic on the very next move, misreading, ignoring or forgetting the Opponent's plans allowing them to succeed etc.
More subtle errors which lead to Won games include Strategic mistakes such as Not Castling in time, Exchanging Off the wrong Minor Piece, Making Threats when the key position is elsewhere on the Board, placing reliance on a Defence which is vulnerable in reality and so on.
Conversely, not taking advantage of a Blunder or Mistake is as just as disastrous.
Such obvious opportunities must not be squandered especially as some Chess games are won by the tiniest of margins.