Captain Majendie would restrict the use of the word blunders to those aberrations of human behaviour which cannot be attributed to any avoidable cause. For my purpose this definition is unduly narrow and I use the concept in a slightly wider sense, namely: ‘errors, or aberrations, without apparent cause’. An understanding of the underlying causes of blunders is essential to their proper study, and to the reduction of blunder rates. Assuming that these causes are factors of human behaviour, there should be some connection between the occurrence and causes of blunders in different fields of human activity. In the hope that this may be so, I summarize the conclusions drawn from investigating blunders over a period of 25 years with a staff of 20–30 engaged on numerical computation and proof-reading.