The triple collocation technique, which retrieves the error variances of three sets of measurements of the same parameter, is applied to soil moisture records in central Spain: ASCAT remote sensing observations, REMEDHUS in-situ probes, and the ERA Interim model. The objective is the estimation of the temporal variability of the error of ASCAT. The three data sets have to be calibrated with respect to each other as they show different mean values and dynamic ranges. The time-variant estimation of both the error and the calibration parameters is shown to be very sensitive to the extents of the temporal windows used and the calibration procedure. Due to the temporal fluctuations of the calibration constants, artefacts such as seasonal variations and extreme values are introduced. This case study shows that the temporal analysis of the errors using the collocation technique can lead to spurious results when the data sets have to be referenced with respect to one another.