Byes given out at random
A bye should go to a top seed. If you hand it to a low seed, that team gets a free win and the top seed has to play, which defeats the point of seeding.
Top seeds meeting too early
Seed 1 and seed 2 must be on opposite halves. Seeds 1 to 4 must be in different quarters. Copying a bracket template from another size breaks this; use a bracket made for your team count.
Forgetting the grand final reset
In double elimination, if the losers-bracket team wins the first grand final, a second game is required. Without it, two teams with one loss each would be separated by a single game. Put the extra game on the schedule, marked "if needed": game 15 for 8 teams, or 2n − 1 for any n.
Losers bracket rematches
If losers drop into the wrong slots, teams that just met in the winners bracket meet again at once. A correct drop order keeps them apart for as long as possible.
Playing games out of order
A game cannot start until both of its teams are known. Number the games in the order they can be played and call them in that order. The numbered games on this site are in that order.
Counting the games wrong
A single elimination bracket of n teams has exactly n − 1 games, and a double elimination bracket has 2n − 2, plus one if the final is replayed. If the number of games on your sheet is different, a game is missing or a slot has been used twice. Count them before the first game starts.
Questions
How do I check a bracket by hand?
Count the games against the formula, check that seeds 1 and 2 are on opposite halves, and confirm that every team appears in exactly one first-round slot or has a bye.