This study focuses on methodological errors that arise when firm valuation is carried out in practice. Violation of assumptions underlying the valuation models are examples of methodological errors. We analyze valuation spreadsheets from five Danish financial institutions (i.e., stockbrokers and corporate finance departments) in order to trace if firm valuation models are properly applied. We conclude the following: (i) Methodological errors often cause valuation models to generate estimates that differ significantly from the theoretically correct value; and (ii) Firm value estimates were biased due to a variety of methodological errors. The implications of those errors may be significant. Investors are exposed to poor recommendations. Financial institutions such as investment bankers and stockbrokers may be exposed to bad reputation and lawsuits. Accounting firms that do not carry out firm valuation correctly (for example in testing goodwill for impairment) also run the risk of litigations.