We should first define what we mean by errors, which are technically different from mistakes. Mistakes are typically made due to tiredness or misunderstanding of a question, rather than through any lack of knowledge. Errors on the other hand are due to fundamental limitations of knowledge. Errors can be classified as, in a sense, developmental errors, where the student makes incorrect assumptions about the language using their current knowledge. They are different from mistakes as mistakes can be self corrected, whereas errors can not.