There are too many variables to even try and diagnose this without being there. How old is your house? Does it have copper or aluminium wiring? What brand electrical panel do you have? If you were a little closer to me I would come look at it for you.
I would hope that the receptacle circuit the humidifier is plug into is not on the lighting circuit. Nothing in the Code says it cant,Well with exception to "appliance circuits and bathroom receptacle circuits. I just prefer to not do it that way. Often times the receptacle circuits get overloaded and I hate for the breaker to trip and take the lights out also. I assume neither of those are the case. But I would still rule that out first. Maybe check the connections behind the receptacle.
If the receptacles are wired using the push in connections first thing I would do (after checking the breaker and turning it off) is to put the wires on the receptacles under the screws. I hate thoes push in contacts and have seen many that made poor contact. I am not an electrician and only work on my own stuff. If I have ran into enough bad contacts with push in connections to take note they can't be all that good an idea in the first place.