Uninsulated can lights are a common source of attic heat.
Leaks around bath ceiling fans introduce moisture as well as heat, as evidenced by mold on the sheathing in this attic.
In one home, a disconnected vent pipe led to icicles inside the attic as well as ice dams on the roof.
Frost can build up in the attic even when there is no ice on the roof; this is an insidious problem, as it can take weeks to detect.