In A Clash of Kings, the Appendix lists under House Frey:
Emmon's son, SER LYONEL, m. Melesa Crakehall
And earlier in page 486, Arya's chapter, she runs an errand for a Ser Lyonel:
"Weasel," Weese said that afternoon. "Get to the armory and tell Lucan that Ser Lyonel notched his sword in practice and needs a new one."
Arya doesn't know the man, so I'm not sure that the Ser Lyonel in Harrenhal is the same as the Ser Lyonel Frey mentioned in the Appendix, Lord Walder's grandson. Is it possible, i.e. were there Freys in Harrenhal at this point in the series?