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The paper comprises the calculations of amplitudes and phases of tidal harmonic constituents, performed on hourly sea level data recorded at the Split tide gauge in the period 1957-2001. Interannual changes in all constituents have been detected, stronger in phases than in amplitudes. For example, the estimated change in M2 amplitude and phase is 22% (1.31 cm) and 24.9° between the 1962–1978 and 1957–1961 periods, respectively. Some of the differences are generated artificially throughout the measurements (clock errors, positioning and stretching of a chart) and within the digitising procedure, rather than by natural processes and changes (e.g. changes in mean sea level). This is the reason why the M2 and K1 amplitudes were recomputed with 3–4 mm larger values using newer software, thereby decreasing their standard deviation by 60–70% in the 1986–1995 period. Artificial errors may be reduced by the upgrading of digitising software; however, most of the errors still remain in the series. These errors may have repercussions when trying to explain some unusual findings: the energy of de-tided sea level series at the M2 tidal period (12.4 h) has been assumed previously to be a result of nonlinear coupling, but it may be caused, at least partly, by timing errors in the time series.