If ever a technology encouraged extension cords between rooms, this is it. We've had problems with lights, but usually related to the switch or a bulb that burns out. Plug-in devices that trip an AFCI are appliances with built-in switches or thermostats, like irons, heaters and vacuums.
Since you had a problem in the evening, it makes me wonder about light bulbs. Are any CFL lamps or fluorescent ballasts involved?
