Beware of commonly used approximations. Part I. Errors in forecasts

Funding for this work was partially provided by the Spanish MINECO under projects AYA2014- 58747-P AEI/FEDER, UE, and MDM-2014-0369 of ICCUB (Unidad de Excelencia Maria de Maeztu). NB is supported by the Spanish MINECO under grant BES-2015-073372. JLB is supported by the Allan C. and Dorothy H. Davis Fellowship, and has been supported by the Spanish MINECO under grant BES-2015-071307, co-funded by the ESF during part of the development of this work. AR has received funding from the People Programme (Marie Curie Actions) of the European Union H2020 Programme under REA grant agreement number 706896 (COSMOFLAGS). GS was supported by the Erasmus+ for Trainership grant during the early stages of this work, subsequently by grant from the “Maria de Maeztu de Ci`encies del Cosmos” project mentioned above. GS is supported by the INFN INDARK PD51 grant. LV acknowledges support by European Union’s Horizon 2020 research and innovation programme ERC (BePreSySe, grant agreement 725327).