Non-parametric reconstruction of the dark energy equation of state ( EoS ) aims to determine the EoS as a function of redshift without invoking any particular dark energy model , so that the resulting EoS can be free of model-induced biases or artifacts .
Without proper regularization , however , such reconstruction is often overwhelmed by the noise of poorly constrained modes .
An intuitive regularization scheme is to assume a priori the dark energy EoS to evolve at most slowly with time , which may be enforced by a correlation between the EoS at different epochs .
Indeed , studies that impose the correlation prior are able to significantly reduce the uncertainties of the reconstructed EoS and even show hints for dynamical dark energy .

In this work , we examine the correlation prior using mock datasets of type Ia supernovae ( SNe Ia ) , baryonic acoustic oscillations ( BAOs ) , age-derived Hubble parameter , Hubble constant , and cosmic microwave background .
We find that even though the prior is designed to disfavor evolving equations of state , it can still accommodate spurious oscillating features at high significance .
Within the 1000 mock datasets of existing observations that are generated for the concordance cosmological model , i.e. , the input dark energy EoS w = -1 , there are 688 ( 69 ) cases recovering an EoS that departs from -1 by more than 1 \sigma ( 2 \sigma ) in one or more redshift bins .
The reconstructed EoS turns up and down markedly in many cases .
Moreover , inverting the signs of the randomly assigned errors of the mock data more or less reverses the behavior of the EoS .
Spurious results occur even more frequently when idealized SN Ia and BAO data from future surveys are included .
Our tests suggest that further studies are needed to ensure accurate reconstruction of the EoS with the correlation prior .