Later on when you start a tribe, tribemates can come to ask you for favors, but tehy can ask you for infinite favors...so effectively clanmate A could ask you for say, 10 favors in a row without paying you back unless you chase them for it after each time.
Also, another bug, if you give gifts to a clan you owe favors to, they will say the favor has been repaid, but its not, the game still counts you as oweing a favor to them.