Is there any load and balance and attitude that can put positive G on one wing and negative on the other? I got caught in gusty cross wind on the  final approach to Catalina airport last week and decided to add power and push for quick turn to come back over laterally sliding runway in a Cessna 172N. Later I was thinking back about the distribution of wind and lift pressure on the wings and could imagine a loading that would be directed upward under my left wing but changing downward on at least part of my right wing. Imagine that you're viewing the plane from behind and see it like a catamaran banked into 30' angle. And a horizontal line not necessarily passing through center crossing it as the sea water level. Is it possible and what to do to handle it?
Just imagine an aerobatic aircraft going straight up and rolling at the same trime - sure it will have positive lift on one side and negative on the other while accelerating into the roll or when stopping the rolling motion. Note that during a constant rolling motion while in a vertical climb the overall rolling moment and the overall lift both are zero.

Also, wing twist will cause very little lift or even a small downforce at the wingtip at low angle of attack. I remember that the wingtips of the SB-8 and SB-10 gliders would point downwards from the negative lift at high speed. The high aspect ratio glass fiber wing would bend a lot and indicate the local lift with a local change in curvature.

However, if you were flying an approach, the aircraft should have flown too slowly to allow for that condition, unless it also started to roll violently from the lift asymmetry. If you had to apply full aileron to counter a gust-induced rolling moment, that very aileron deflection would avoid any negative lift on one of the wingtips. This only leaves the possibility that you had negative lift for a very short moment and over a small part of the wing when the gust hit you and before you countered the rolling motion with the ailerons.