Stall. :)
Think about it. A stall is when the angle, at which the airfoil travels through the air, becomes so great that the air will not flow "evenly" over the top of the wing but instead will create a lot of vortices and turbulent flow behind the wing edge (well there usually is no one angle at which this suddenly happens, what is usually the case is that AFAIK this zone tends to move further towards the front of the wing, starting out at the back, and the less area of the wing that has laminar flow, the less lift it will generate). Usually when you sit in an actual airplane and pull g's, you will (depending on how rapidly this sets on in the particular type of AC) at a certain point notice that if you pull any harder on the stick, g force will get less.
And that is the point. Stall is caused by angle of attack. What influences angle of attack? Speed and gs you pull (also, plane weight, but that is secondary in out little thought experiment). The more gs you want to pull, the more speed you need so you don't enter a stall first. The slower you go, the less gs you can pull until eventually, you "run out of gs" that is, your airplane can't even sustain 1 g without a stall. That is why there is an infinite number of stall speeds, depending on the load you pull.