In recent years, the debate on flexible employment has been at the center of political and scientific discussion in Europe. Findings on mobility from flexible to permanent employment can be severely biased due to measurement error, usually present in the data used for analysis. This paper uses a mixture hidden Markov model (MHMM) to study the effect of measurement error on the types of flexible employment careers that exist in the labour market. Specifically, we employ an MHMM with two indicators for the employment contract, coming from linked data from the Labour Force Survey and the Employment Register of the Netherlands for the period 2016–2019. Our results show that measurement error biases considerably the trajectories we find.