Deflection of a control surface on a trailing edge alters the camber.. Down, increases the camber, and the pitching moment.
And for a surface such as an aileron, this means the wing ahead of the surface has "wash-in".
Deflect the surface up, the camber changes, but the pitching can be decreased... and the wing ahead of the surface has "wash-out".
Wash-in makes the plane more susceptible to tip-stalls, as the angle of attack, AOA, as the Caffeeman mentions increases. If the plane is already near the stall angle of attack, the change is frequently enough to push the AOA past the stall point. The wing on that side stalls.. and the plane rolls over... as the wing on the other side is not stalled, it lifts, and the plane rolls -opposite- the command direction. towards the stalled side. This leads to confusion, as more aileron in the same direction doesn't fix the problem.
Wash-out OTOH reduces the chance of a tip-stall, as the AOA for that part of the wing is decreased, and the center section will stall first, which is a "good" condition.
For outboard mounted surfaces, therefore, spoilerons are better than flaperons..
When used -with- flaps, flaps down and spoilers/ailerons up is commonly used on high performance gliders, in the "CROW" mode. Adds LOTS of drag, without increasing speed, for precision landings.
Flaperons have another problem.. When both are down as far as they can go, they lose the "aileron" mode. Turning now must be done with the rudder, if the flaperons are really down. When commanding a right turn, say, with flaperons, the right aileron goes up, as it should. The left aileron -should- go down, but it's already as far down as it can get. The result is the drag on that side doesn't change.
On the other side though, both the drag and lift decrease as the surface goes up. The result is much like a tip-stall; the plane turns -opposite- the commanded direction. Disorienting and confusing when it's first experienced.
The spoileron situation though is benign.. the spoiler that's as far up as it can go on the right stays up when a turn is commanded .. the drag doesn't change.
The spoileron on the other side now moves down, decreasing drag on that side, and the plane turns into the expected direction.