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The NRAO VLA Sky Survey (NVSS) Rotation Measure (RM) catalogue is invaluablefor the study of cosmic magnetism. However, the RM values reported in it can beaffected by n$\pi$-ambiguity, resulting in deviations of the reported RM fromthe true values by multiples of +-652.9 rad m-2. We therefore set off toobservationally constrain the fraction of sources in the RM catalogue affectedby this ambiguity. New broadband spectro-polarimetric observations wereperformed with the Karl G. Jansky Very Large Array (VLA) at 1--2 GHz, with 23n$\pi$-ambiguity candidates selected by their peculiarly high |RM| values. Weidentified nine sources with erroneous RM values due to n$\pi$-ambiguity and 11with reliable RM values. In addition, we found two sources to be unpolarisedand one source to be inconsistent with neither n$\pi$-ambiguity nor reliable RMcases. By comparing the statistical distributions of the above two mainclasses, we devised a measure of how much a source's RM deviates from that ofits neighbours: $\Delta/\sigma$, which we found to be a good diagnostic ofn$\pi$-ambiguity. With this, we estimate that there are at least 50 sourcesaffected by n$\pi$-ambiguity among the 37,543 sources in the catalogue.Finally, we explored the Faraday complexities of our sources revealed by ourbroadband observations.

A Broadband Spectro-polarimetric View of the NVSS Rotation Measure Catalogue I: Breaking the n{\pi}-ambiguity