Lord of the crossing
From A Wiki of Ice and Fire
Lord of the crossing is a game played at the Twins by House Frey; the "Lord of the Crossing" is one of the titles used by Lord Frey, as the bridge at the Twins crosses the Green Fork. The game usually involves lots of hitting and arguing. The purpose of the game is to be the lord of the crossing, usually a player standing on a bridge over water.
The game must be played on some sort of bridge placed over water of some sort. One player, the "lord of the crossing," stands in the middle of the bridge with a staff. When another player approaches, the lord of the crossing must say "I am the lord of the crossing, who goes there?" The player who approaches must then present his reasons for crossing the bridge and why he should be allowed to cross. The lord asks the player questions and makes them swear oaths. The player does not have to respond truthfully to the questions but the oaths are binding unless the player says "Mayhaps" quickly enough that the lord does not notice. Then the player must attempt to knock the lord off the bridge. The lord can knock a player into the water at any time, and he is the only one armed with a staff. Only when the lord is displaced can another player become lord, but only if they said mayhaps in the game, otherwise it means immediate disqualification.
A Clash of Kings
A Storm of Swords
At the Twins prior to the wedding of Roslin Frey and Lord Edmure Tully, the embittered Lord Walder Frey speaks to King Robb Stark, saying "You wanted crossing and I gave it to you, and you never said mayhaps, heh." Robb is unaware of the rules of the game, however. Robb is betrayed and murdered during the Red Wedding, orchestrated by Walder as vengeance for Robb marrying Jeyne Westerling instead of a Frey.