FIDE. The "zero tolerance" rule (introduced in 2009) that, unless specified otherwise, a player who is late at all for the start of the round forfeits the game was modified in 2014. The rules of a competition may specify a different "default time" for the tournament. If the rules of the competition allow players to arrive late and both players are late, all the elapsed time comes off White's clock.
USCF. The game is lost by a player who arrives at the chessboard more than one hour late. If both players are late, the elapsed time from the start of the round until the first player arrives is divided in half, and that time is subtracted from each player's clock. (So, for instance, if the first player is forty minutes late, twenty minutes should be subtracted from both sides of the clock.)
FIDE. If claiming a draw by triple occurrence of position or the 50 move rule, you must write your move on the score sheet without making the move on the board. If you make the move on the board, you are no longer considered to be "on the move" (even if your clock is still running), and a draw claim will be rejected. (Also see item 2 in "obscure differences" below.)
USCF. If claiming a draw by triple occurrence of position and the player's next move would cause the third occurrence, the correct procedure is to write the move on the scoresheet but not to execute the move on the board. However, under US Chess rules, a player is considered to be "on the move" until the player presses the clock. So, if the player (incorrectly) makes the move on the board but does not press the clock, the player does not lose the right to claim the draw. Also, to claim a draw by the 50 move rule, the player should make the move on the board (if needed to reach the count of 50 moves by both sides) and must claim the draw before pressing the clock.
FIDE. Score sheets must be brought up to date at the end of a non-sudden death time control. If one player must complete his score sheet, he does so while his clock is running before he makes a move in the new time control. If both players must complete score sheets, the arbiter may assist, and both clocks are stopped during the reconstruction.
USCF. The director may waive the requirement to bring scoresheets up to date at the end of a non-sudden death time control. In practice, this requirement is almost never enforced.
FIDE. When promoting a pawn, the choice of promotion piece is final as soon as the piece touches the promotion square (even if the player has not yet released the promotion piece).
USCF. When promoting a pawn, the choice of promotion piece is final as soon as the piece touches the promotion square (even if the player has not yet released the promotion piece). (This changed to match the FIDE rule as of 1 January 2018.)
FIDE. An inverted rook is not allowed to be used as a "queen" when promoting a pawn. If a promotion piece is not readily available, the player must stop the clocks and ask the arbiter for assistance. An upside-down rook is treated as promotion to a rook. If the arbiter sees this, he will turn the rook right side up and the game continues.
USCF. The US Chess rules explicitly state that, when promoting a pawn, an upside-down rook is to be considered to be a queen.
Insufficient Losing Chances
FIDE. A player with less than two minutes remaining on the clock may claim a draw under Article G.5 (the vague equivalent of US Chess rule 14H, "insufficient losing chances"), even if there is a delay clock already in use for the game. Article G.5 does not apply if the time control uses any increment.) The player may claim a draw based on the opponent not making any attempt to win the game by normal means. Article G.2 requires that it be announced before the start of an event whether appendix G applies to the event.
USCF. US Chess rule 14H (insufficient losing chances) does not apply if the time control includes either delay or increment and the game is played with a properly set delay or increment capable clock.
FIDE. It is possible to lose on time in situations that are a draw under US Chess rules. For instance, GM Friedel (IM Friedel at the time) lost on time with a king and rook vs. king and knight. Under the FIDE laws of chess, the game is drawn when one player runs out of time only if there is no legal sequence of moves by which the opponent could checkmate the player. Since there is a helpmate that allows a king and one knight to checkmate a player with a king and rook, GM Friedel lost the game. (Note: Earlier versions of this page incorrectly attributed this loss to GM Nakamura. The game in question is IM J. Friedel - GM S. Halkias, Bad Wiessee Open, Germany, 2007. See this story on Chess Life Online for more details.)
USCF. US Chess rule 14E (insuffient material to win on time) specifies cases where the game is drawn even if one player runs out of time. One of the cases listed in rule 14E is the opponent having only a king and knight (and not having a forced win).
Illegal Move Time Adjustment
FIDE. When correcting illegal moves, the arbiter may use his best judgment to determine the time on each clock. (This includes the right of the arbiter to choose not to adjust the clocks at all.)
USCF. In the case of an illegal move, there is no adjustment of the times shown on the clock.
Illegal Starting Position
FIDE. If the players start the game with the colors reversed, before ten moves have been made by both players, the game continues unless the arbiter rules otherwise. (After ten moves, the game continues with no change.) If the game started with the pieces incorrectly set up, the game is canceled and a new game played in its place, regardless of how many moves have been made when the error has been noticed. (While it is a common fix, it is incorrect to fix a reversed king and queen [the most common case of an incorrect initial position] by just swapping the king and queen back to their correct squares.)
USCF. Starting the game with the colors reversed is treated as an illegal move at the start of the game, as is starting the game with the pieces set up incorrectly. In both cases, the mistake must be corrected within the first ten moves by both players, or the game will continue as is. (While it is a common fix, it is incorrect to fix a reversed king and queen [the most common case of an incorrect initial position] by just swapping the king and queen back to their correct squares.)
Tournament Director Draws
FIDE. The game is drawn without either player making a claim if the same position occurs on the board after five occurrences of the same position with the same player on the move each time and the same possible moves each time ("five-fold repetition") or if both players have completed 75 moves without a capture or a pawn move.
USCF. The TD may declare the game drawn if the same position occurs on the board after five consecutive pairs of moves for each player ("five-fold repetition") or if both players have completed 75 moves without a capture or a pawn move. However, the draw is not automatic, and the TD is not obligated to call the draw. (This is a weaker form of the FIDE rule.)