Pipeline bubbling. Eliminating hazards. This error occurs because Instruction 2 reads Register 1 before Instruction 1 has committed/stored the result of its write operation to Register 1. This article needs additional citations for verification. A structural hazard occurs when a part of the processor's hardware is needed by two or more instructions at the same time. Bubbling the pipeline, also termed a pipeline break or pipeline stall, is a method to preclude data, structural, and branch hazards. scoreboarding, in which case a pipeline bubble is needed only when there is no functional unit available the Tomasulo algorithm, which uses register renaming, allowing continual issuing of instructions .