Two separate “reference” wires were arranged to act as an X-wire. First, the effects of the orientation of one of the wires with respect to the other, and of both wires with respect to the mean flow, were investigated. Then, components of typical hot-wire probes (stem, prongs, . . .) were being advanced toward the reference wires. The analysis of the response of the reference wires during the advancement uncovered (a) a perturbation induced on the RMS value v′ of the transverse component of the velocity fluctuation and on the Reynolds stress uv, (b) changes of the wire sensitivities, and therefore also (c) errors affecting the v′ and uv measurements performed with typical probes.