The paper discusses the problem of measuring velocity fluctuations of a turbulent boundary layer using single and X-wire probes. It seems that the difference between the streamwise fluctuating component of these two probes results not only from spatial resolution, but also from influence of the wall-normal fluctuating component, which is usually not considered. It was shown that the vector summing these two components obtained from X-wire probe gives the shape of fluctuation distribution obtained from a single-wire probe. It implies that the underestimation of the near-wall peak of streamwise fluctuating component in X-wire measurements results from disregarded wall-normal fluctuations, which is obviously taken in the case of a single-wire probe. Moreover; it was shown that the criteria for wire length i.e. $ l^+\leq20$ could not be sufficient to properly estimate the streamwise and wall-normal fluctuations.