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When a Skylake CPU makes an incorrect prediction regarding a branch, it causes a pipeline stall. The pipeline is the series of stages that a CPU instruction goes through to be executed, from fetching the instruction from memory to retrieving the necessary data and writing the results back. If the CPU mis-predicts a branch, meaning it predicted the wrong path for the program to follow, the instructions that were fetched in the wrong prediction must be discarded, and the correct instructions need to be fetched, causing a delay or a stall in the pipeline. This can result in a reduction in performance or efficiency.