For maximum performance, the RT FIFO VIs do not check for existing errors. To prevent execution with an error present:
LabVIEW 8.0 and prior: You can modify the RT FIFO VIs to use a case structure that examines an error input. If an error occurs, then one case can simply pass the error through the case structure without executing any other code. If no error occurs, then the other case can execute your code.
LabVIEW 8.2: RT FIFOs are LabVIEW primitive functions in LabVIEW 8.2 and newer. Primitives are VIs that have been optimized for performance, and do not have an exposed block diagram. The previous workaround must be implemented in a separate subVI.
LabVIEW 8.6 and later: Real-Time FIFO VIs have been replaced with Real-Time FIFO Functions. Real-Time FIFO Functions do not ignore errors and provide standard error in functionality.