Slotted multi-element configurations are widely used because they are very effective in increasing the maximum lift of airfoils during takeoff and landing. However, when stall occurs in the leading-edge region of one of the elements, the outcome is a sudden and dangerous drop in performance of the whole configuration. The results of the experimental verification of a computer-designed single-slotted flap are presented. Many realistic configurations, including the computed optimum, were tested and compared to the numerical predictions. For a number of these configurations, sudden leading-edge stall due to the bursting of a laminar bubble was detected on the flap, followed by severe stall hysteresis. Present numerical design codes do not help the designer much in predicting this important phenomenon, and wind-tunnel testing remains necessary. Therefore, there is a need for better bubble bursting prediction methods. A possible direction for improvement is discussed, focusing on the unsteadiness of the flow.