Parsing systems which rely on hand-coded linguistic descriptions can only perform adequately in as far as these descriptions are correct and complete. The paper describes an error mining technique to discover problems in hand-coded linguistic descriptions for parsing such as grammars and lexicons. By analysing parse results for very large unannotated corpora, the technique discovers missing, incorrect or incomplete linguistic descriptions. The technique uses the frequency of n-grams of words for arbitrary values of n. It is shown how a new combination of suffix arrays and perfect hash finite automata allows an efficient implementation.