When Robb arrives at the twins before the red wedding and asks if he can cross the river, Walder says
'you wanted crossing and I gave it to you and you never said mayhaps, heh'..
Then when catelyn asks for food to secure the guest right, Walder says
'.... A loaf of bread, a bite of cheese, mayhaps a sausage.'
The mayhaps is a reference to the Lord of the Crossing game that little Walder and Big Walder play at Winterfell. In the game, someone is the lord of the crossing and he decides whether or not someone gets to cross. He may make the would be crosser answer questions or swear a series of oaths. The person must obey the oaths UNLESS they sneak in the word MAYHAPS - if they do without the Lord noticing then the oath is not binding.
Walder says Robb never said mayhaps when he wanted crossing, which in reference to the game allules so his promise saying he would marry a Frey girl could not be broken.
Frey then says mayhaps when promising the guest right to Robb which lets him break the guest right and start the red wedding.
To add onto this. Wyman ******* Manderly does the call back of a lifetime when he says:
"So young," said Wyman Manderly, "Though mayhaps this was a blessing. Had he lived, he would have grown up to be a Frey.
This quote is in reference to when Manderly hears about Little Walder dying so young.
Edit: this is not an unwritten Rule in Westeros, its just an allusion by George